When you have both Medicare and employer-sponsored health insurance, your employer plan is usually the primary payer, meaning it pays your medical bills first. Medicare then picks up some or all of the remaining costs as secondary coverage. But this depends on the size of your employer, how you qualified for Medicare, and whether you’re still actively working. In several common situations, the order flips and Medicare pays first.
The 20-Employee Rule for Age-Based Medicare
If you’re 65 or older and still working, the key factor is how many people your employer has on staff. At companies with 20 or more employees (counting both full-time and part-time workers), your employer’s group health plan pays first and Medicare pays second. This is the most common scenario for people who keep working past 65.
At companies with fewer than 20 employees, the order reverses. Medicare becomes your primary coverage, and your employer plan pays second. This matters because a small-employer plan that pays second may cover less than you expect. If you work for a small company and are approaching 65, it’s worth checking with your benefits administrator to understand how your plan coordinates with Medicare, since your out-of-pocket costs could change significantly.
There’s a wrinkle for multi-employer plans, where several companies pool together to offer coverage. If even one employer in the group has 20 or more employees, the plan generally treats everyone under the working-aged rules, meaning the group plan pays first. However, a multi-employer plan can apply for an exception from CMS for workers tied to a specific small employer within the group. This exception has to be formally approved and applies only to named individuals.
Different Rules for Disability Coverage
If you qualify for Medicare through a disability (rather than age), a separate threshold applies. The cutoff jumps from 20 employees to 100 employees. At companies with 100 or more workers, your employer’s large group health plan pays first and Medicare pays second. If your employer has fewer than 100 employees, Medicare is primary.
The same multi-employer logic applies here. If multiple employers contribute to a single plan and at least one of them has 100 or more employees, the plan pays first for everyone in the group, regardless of which specific employer you work for.
End-Stage Renal Disease Has a 30-Month Window
People who become eligible for Medicare because of permanent kidney failure (end-stage renal disease) face a unique coordination period. For the first 30 months after Medicare eligibility begins, your employer or union group health plan continues to pay first. Once that 30-month window closes, Medicare becomes primary and your employer plan shifts to secondary. A new 30-month coordination period starts each time you sign up for Medicare based on kidney failure.
When Medicare Always Pays First
Several types of coverage are not considered “group health plans based on current employment,” and Medicare pays before all of them:
- Retiree coverage. Medicare pays first, and your former employer’s retiree plan pays second. If you’re eligible for Medicare but haven’t enrolled, your retiree plan might not pay your medical costs at all during that gap.
- COBRA continuation coverage. If you’re eligible for Medicare, COBRA may only cover a small portion of your bills. You could end up responsible for most costs yourself.
- Individual marketplace plans. Coverage purchased through the Health Insurance Marketplace is not employer group coverage for coordination purposes.
- VA benefits. Veterans Affairs coverage doesn’t count as current-employment group coverage either.
The distinction between active employment and these other categories is critical when you’re deciding whether to enroll in Medicare or delay it.
Why Getting the Timing Right Matters
If your employer plan legitimately pays first because you’re actively working at a company that meets the size threshold, you can safely delay enrolling in Medicare Part B without penalty. Once your employment ends or you lose that employer coverage, you get a Special Enrollment Period of eight months to sign up for Part B. No late penalty applies as long as you enroll within that window.
The penalty for missing this deadline is permanent. You’ll pay an extra 10% on your Part B premium for every full 12-month period you could have enrolled but didn’t. Someone who waits two years too long, for example, pays a 20% surcharge on every monthly premium for the rest of their time on Medicare.
COBRA coverage does not protect you from this penalty. Even though COBRA continues your old employer plan, it doesn’t count as coverage based on current employment. Your eight-month Special Enrollment Period starts when you stop working or lose your employer coverage, whichever comes first, not when COBRA ends. If you rely on COBRA for two years thinking you’re covered, you could face both a penalty and a gap in coverage.
Health Savings Account Implications
If you’re contributing to a Health Savings Account through your employer’s high-deductible plan, enrolling in any part of Medicare ends your eligibility to contribute. Starting with the first month you’re enrolled in Medicare, your HSA contribution limit drops to zero. This applies retroactively too. If you delay your Medicare application and your enrollment is later backdated, any HSA contributions made during that retroactive period count as excess contributions and may trigger tax penalties.
This creates a real tradeoff for workers over 65 with HSA-eligible plans. Staying on employer coverage and delaying Medicare lets you keep contributing to your HSA, but you’ll want to stop contributions at least six months before applying for Medicare Part A, since Part A can be backdated up to six months.
How to Check Your Specific Situation
The quickest way to determine your payment order is to answer three questions: Are you still actively working (not retired, not on COBRA)? How many employees does your company have? And did you qualify for Medicare through age, disability, or kidney failure? Your answers map directly to the rules above. Your employer’s benefits office or your plan’s benefits coordination department can confirm which payer is primary, and it’s worth verifying this before you receive care rather than sorting it out after a claim is denied.