Medicare Coordination of Benefits: Who Pays First (2026)

When you have Medicare plus another plan, getting the order of payment wrong costs real money, and one common COBRA mistake can trigger a penalty that lasts as long as you have Part B. Medicare coordination of benefits is the set of rules that decides which of your plans pays first, whether that's your employer, a retiree plan, COBRA, or Medicare itself.

How Medicare coordination of benefits works: primary vs. secondary payers

When you have Medicare and other coverage at once, one plan is the "primary payer" and the other the "secondary payer." The primary payer processes your claim first, up to the limits of its coverage; whatever remains, including deductibles, coinsurance, and anything it didn't cover, then goes to the secondary payer.

Which plan gets which role is the hard part: the same two plans swap places depending on your age, whether you're still working, how many employees your employer has, and whether your Medicare came from age or a medical condition.

If you're unsure which plan pays first in your own situation, contact your plan or call the Benefits Coordination & Recovery Center at 1-855-798-2627 (TTY 1-855-797-2627).1

Employer coverage: the employee count changes everything

Two questions decide this, not one: what kind of plan you have, and how many employees your employer has. And the count is not the same for everyone: 20 if your Medicare comes from being 65 or older, 100 if it comes from disability before 65.1

Large employer plans (20 or more employees): If you're 65 or older and actively working, your employer has 20 or more employees, and your group health plan is not a tribal health plan, your employer group health plan pays primary. Medicare pays secondary. The same order applies if your employer is part of a multi-employer group health plan in which at least one company has 20 or more employees, unless that plan has elected the small-employer exception for your employer, in which case Medicare pays first, so ask the plan which order applies to you.1 If you delay Part B during this period, that's generally allowed without penalty, because you have qualifying current employer coverage.1 One warning travels with this branch: if the plan paying first is an HMO or an employer PPO and you get services outside its network, it's possible that neither the plan nor Medicare will pay, so call the plan before you go out of network.1

Small employer plans (fewer than 20 employees): If your employer has fewer than 20 employees, Medicare becomes the primary payer, and the employer plan pays secondary. Medicare's who-pays-first rules confirm the threshold.1 Because Medicare is the primary payer here, you typically should enroll at 65 to avoid gaps in coverage.1

Tribal health plans: If your health insurance is through a tribal health plan, the order runs the other way from the 20-or-more-employee rule above. Medicare pays first and the tribal health plan pays second. If you get drugs through an Indian health facility, you'll continue to get them at no cost to you, and your coverage won't be interrupted.1

If your Medicare comes from disability rather than age, the threshold is 100 employees, not 20. If you're under 65, entitled to Medicare because of disability, and covered through your own or a family member's current employment, the plan counts as a "large group health plan" and pays primary, with Medicare secondary, when at least one employer in the plan employed 100 or more full-time and/or part-time employees on 50 percent or more of its business days in the previous calendar year. Below 100 employees Medicare generally pays first, unless the plan is a multi-employer plan in which at least one participating employer has 100 or more employees, in which case Medicare stays secondary.4 Put concretely: at a 50-employee company, your employer plan pays first if your Medicare came from turning 65, and Medicare pays first if it came from disability.1

All of that applies to active employment. Once employment ends, the rules shift.

Per the Medicare.gov working-past-65 guide, you can delay Part B without penalty while you (or your spouse) are still working and covered by a group health plan based on that current employment. Not every job-based plan counts: Medicare gives this guidance for group health insurance that is available to everyone at the company, and tells people who are self-employed, or whose plan is not available to everyone at the company, to ask their insurance provider whether it is employer group health plan coverage as the IRS defines it, and to sign up at 65 if it is not. You then have an 8-month Special Enrollment Period, and the sources state its start a month apart: CMS counts from the month employment or that coverage ends, whichever comes first, Social Security from the month after. Count from the earlier one. Coverage generally starts the first month after you sign up, and if you sign up while still working, or within the first full month after the employer coverage ends, you can ask to delay your Part B start date by up to 3 months. This Special Enrollment Period does not apply to people eligible for Medicare based on End-Stage Renal Disease (ESRD), who have a different route, covered below.2

Retiree coverage: Medicare pays first

If you have coverage through a former employer's retiree plan, Medicare generally pays primary and the retiree plan pays secondary. When you become eligible for Medicare, you may need to sign up for both Part A and Part B to get full benefits from your retiree coverage, and that coverage might not pay your medical costs for any period when you were eligible for Medicare but didn't sign up.1

That last point is what costs money. If you were eligible for Medicare but didn't sign up, your retiree plan might not pay, and any balance the secondary payer doesn't cover can fall to you.1

Let your retiree plan know when you enroll in Medicare, and ask whether it needs anything from you so it processes claims in the right order.

The COBRA trap, explained in detail

COBRA is continuation coverage: when you leave a job that offered group health insurance, it lets you stay on that same plan temporarily if you pay the premiums yourself. It feels like a natural bridge into retirement, and that is the trap, because COBRA is not treated as "current employer coverage" for Medicare enrollment purposes.

You retire at 66, still covered by your employer's group plan, so your 8-month Special Enrollment Period starts the month your employment or that coverage ends, whichever comes first. Instead of enrolling in Part B, you elect COBRA, and because COBRA feels like coverage you assume the clock isn't running. It is. Per the Special Enrollment Period rules, the window started when your employment ended, not when COBRA expires, so waiting for COBRA to run out can mean missing it entirely.2

The penalty for missing Part B enrollment without qualifying for a Special Enrollment Period or a Medicare Savings Program, the two exceptions that apply to Part B, is a 10% increase in your Part B premium for each full 12-month period you could have had Part B and didn't, and per Medicare's late enrollment penalty rules, that surcharge lasts as long as you have Part B. The 2026 standard Part B premium is $202.90 a month, and that 10% rides on top of it for as long as you have Part B.5,6

Missing that window closes the working-past-65 route, but it is not the end of the road, and the General Enrollment Period is not your only remaining option. If, on or after January 1, 2023, an employer, a health plan, or an agent or broker for one told you that you didn't need Part B, there is a Special Enrollment Period for exactly that: it begins the day you notify the Social Security Administration, ends six months later, and removes the wait and the penalty rather than merely delaying them. It is bounded, and you have to show both halves by documentation or written attestation, that you didn't enroll during an enrollment period you were eligible for based on what you were told, and that the employer, plan, or agent materially misrepresented or gave incorrect information about enrolling in Part B or premium Part A. Form CMS-10797 is the application. CMS lists six further Special Enrollment Periods alongside that one, including for a declared emergency or disaster, release from incarceration, and termination of Medicaid (those three for events on or after January 1, 2023), and 42 CFR 406.24(d) preserves your right to another where a missed period was caused by group health plan coverage being restored before it ended. Only if none of them fits do you wait for the General Enrollment Period, January 1 through March 31, with coverage starting the first day of the month after you enroll.2

If you're on COBRA and approaching Medicare age, enroll in Part B during your 8-month window from the end of employment. Don't wait for COBRA to run out.

COBRA coordination when you already have Medicare

The rules differ when Medicare is already in place before COBRA. If you're 65 or older and have Medicare because of your age, Medicare pays primary and COBRA pays secondary, the same structure as retiree coverage.1 COBRA can still help with costs Medicare leaves to you, but Medicare's primary role doesn't change.

Let your COBRA administrator know when you enroll in Medicare, and ask how it will process your claims once Medicare is in place.

The ESRD exception: COBRA can pay first for up to 30 months

There is one situation where COBRA pays primary instead of Medicare: when your Medicare eligibility is based solely on end-stage renal disease (ESRD), meaning permanent kidney failure requiring dialysis or a transplant. If that's you and you also have COBRA coverage, COBRA pays primary during a 30-month coordination period that begins on the date you are first eligible to enroll in Medicare because of ESRD, for as long as your COBRA coverage overlaps that period. The same rule applies to any group health plan coverage, not just COBRA, regardless of employer size and regardless of whether anyone is still working, retirees included. After the coordination period ends, Medicare becomes the primary payer for all Medicare-covered services, and a separate 30-month coordination period runs each time you sign up for Medicare based on permanent kidney failure. The ESRD rules don't keep COBRA in force, though: a plan may still end COBRA when you become entitled to Medicare if the COBRA law expressly allows it. Ask your plan how long yours will last.3 The rule carries one precondition: it applies only if Medicare was not already your primary payer on the basis of age or disability at the time you became eligible for or entitled to Medicare on the basis of ESRD.1

If you're already enrolled in Medicare, Medicare becomes the primary payer once the 30-month coordination period ends.3 Confirm with your plan that the switch happened, so a claim isn't sent to the wrong payer. If you deferred Part B during the coordination period, see the enrollment steps below.

Medicare covers ESRD at any age: no age or income test, though there is an insured-status test: you must have worked the required amount of time under Social Security, the Railroad Retirement Board, or as a government employee, or already be getting or be eligible for Social Security or Railroad Retirement benefits, or be the spouse or dependent child of someone who is.3

One thing the 30-month coordination period does not do is reopen your Part B enrollment window: the working-past-65 Special Enrollment Period described earlier does not apply if you're eligible for Medicare based on End-Stage Renal Disease (ESRD), even if you are also working past 65.2

That is not a penalty exposure, though, and acting as if it were is what costs money here. If you apply for Medicare and are approved because of ESRD, you can sign up for Part B without paying a late enrollment penalty. Medicare.gov goes further for people in the coordination period: if your group health plan will pay most or all of your health care costs, you may want to delay signing up until the 30-month coordination period is over, because delaying means not paying the Part B premium for coverage you don't need yet, and your Part B premium won't be higher because you delayed in this situation. Sign up once the coordination period ends, and sign up as soon as you can if the group health plan's benefits are cut or end during it.3 CMS's coordination manual says the same from the other side: people eligible on the basis of ESRD cannot use a Special Enrollment Period, but can defer both Part A and Part B and file an initial application later, usually at the end of the coordination period.1

If your Medicare comes from both age and ESRD, don't assume which rule governs your claims. Confirm who pays first with the Benefits Coordination & Recovery Center, contact details below, before a large bill comes due.1

Medicare coordination of benefits summary: who pays first

The table below gives the order for each situation, and the one thing to watch.1

One caveat spans the last two rows: the 8-month Special Enrollment Period named in the COBRA row does not apply if you're eligible for Medicare based on End-Stage Renal Disease (ESRD).2 For ESRD the route is the one above: no late enrollment penalty, and waiting until the coordination period ends can be cheaper.3

Telling your plans about each other

One practical step is easy to miss: you have to tell your plans when your coverage changes. Nothing prompts you to do it, and each plan only knows what it has on file, so a change you consider obvious, like retiring or picking up Medicare, is invisible to your other insurer until you say so.

When you enroll in Medicare, tell your other insurer. When you retire and lose employer coverage, tell Medicare and any remaining plans. When you elect COBRA, tell that plan whether Medicare is already in place. A stale record can lead to a surprise denial months later.

Behind the scenes this runs through the Benefits Coordination & Recovery Center (BCRC), the CMS contractor that consolidates Medicare's coordination-of-benefits work. Section 111 of the MMSEA requires group health plans, and non-group plans such as liability, no-fault and workers' compensation insurers, to report to CMS so Medicare can coordinate benefits, so the record your plans work from is only as good as what has been reported.4

Two numbers are worth keeping on hand.

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The information on Brevy.com is for educational purposes only and is not a substitute for professional legal, financial, or medical advice. Rules vary by state and program and change frequently. Always verify with the relevant agency or a qualified professional. Brevy is not a law firm, financial advisor, or healthcare provider.