MSP stands for Medicare Secondary Payer, a set of federal rules that determine when Medicare pays second instead of first for your medical bills. When you have other insurance coverage that’s legally required to pay before Medicare, that other coverage is the “primary payer” and Medicare becomes “secondary.” This matters because the order of payment affects what you owe out of pocket and how your claims get processed.
How Primary and Secondary Payers Work
Every time you receive medical care, your insurers need to know who pays first. The primary payer processes the claim and pays its share. Then the secondary payer picks up some or all of what’s left, according to its own coverage rules. If Medicare is your only coverage, it remains your primary payer. But if you also have employer-based insurance, workers’ compensation, or certain other types of coverage, federal law may require that other insurance to pay first.
This isn’t optional. Federal MSP rules override state laws and private insurance contracts. Even if your employer’s health plan says it pays second to Medicare, the MSP provisions still apply when the law designates that plan as primary. The rules exist to protect the Medicare trust fund by ensuring Medicare doesn’t pay for costs that another insurer is legally responsible for.
Employer Coverage and the 20-Employee Rule
The most common MSP situation involves people age 65 or older who are still working (or whose spouse is still working) and have health insurance through that employer. If the employer has 20 or more employees, the employer’s group health plan pays first and Medicare pays second. If the employer has fewer than 20 employees, the order flips: Medicare pays first, and the employer plan pays second.
This threshold is based on the number of people on the employer’s payroll, counting both full-time and part-time workers. For multi-employer plans (like union plans covering workers at several companies), the 20-employee rule applies if at least one participating employer meets the threshold. In that case, the group plan pays first for everyone enrolled, even employees at the smaller companies.
Disability and the 100-Employee Rule
A different threshold applies if you’re under 65 and qualify for Medicare based on a disability. Here, the employer must have 100 or more employees for the group health plan to be primary. If you or a family member currently works for an employer of that size and you’re covered through that job, the employer plan pays first and Medicare pays second.
If the employer has fewer than 100 workers, Medicare is your primary payer. As with the working-aged rule, multi-employer plans trigger the requirement when at least one participating employer hits 100 employees. Self-employed individuals who participate in a large group health plan aren’t counted toward the 100-employee threshold.
End-Stage Renal Disease and the 30-Month Window
People who qualify for Medicare because of end-stage renal disease (ESRD) face a unique timing rule. If you have group health plan coverage when you first become eligible for Medicare through ESRD, your group plan pays first for a coordination period of 30 months. This applies regardless of how many employees your employer has and regardless of whether your coverage is through current employment, COBRA, or even a retirement plan.
Once the 30-month coordination period ends, Medicare becomes your primary payer. During those 30 months, the group plan cannot treat itself as secondary to Medicare, even if its own policy says otherwise.
Workers’ Compensation, Liability, and No-Fault Insurance
Medicare is always secondary to workers’ compensation for job-related injuries and illnesses. If you’re hurt at work and entitled to workers’ compensation benefits, that coverage pays first for any related medical care.
The same principle applies to liability insurance and no-fault insurance. If you’re in a car accident and no-fault auto insurance covers your medical bills, that insurance pays before Medicare. If someone else is at fault and their liability insurance is involved, that insurer is responsible first. This includes situations where a business or individual is self-insured, meaning they carry their own risk rather than purchasing a traditional policy.
These cases often take time to resolve. While a liability or workers’ compensation claim is pending, Medicare can step in and make what’s called a conditional payment to cover your medical bills in the meantime. But “conditional” is the key word: once the primary payer settles the claim, Medicare is entitled to be repaid for whatever it covered.
How Conditional Payments Get Recovered
When Medicare pays conditionally for care that another insurer should have covered, the Benefits Coordination and Recovery Center (BCRC) tracks those payments and handles recovery. Once a settlement, judgment, or award is made, the BCRC identifies which Medicare claims are related and initiates the recovery process. This means a portion of your settlement may go back to Medicare.
For smaller settlements, there are minimum thresholds below which CMS won’t pursue recovery. As of 2025, settlements of $750 or less for physical trauma-based liability insurance, no-fault insurance, and workers’ compensation cases don’t need to be reported, and CMS won’t seek repayment on them.
What This Means for Your Coverage
If you have Medicare plus another form of coverage, the MSP rules determine the payment order for every claim. Getting this wrong can delay your bills or result in denied claims. Your providers and insurers coordinate through a system managed by CMS, but knowing your own situation helps you avoid surprises.
The simplest way to figure out where you stand: if you’re 65 or older and still covered through a current employer with 20 or more workers, your employer plan is primary. If you’re under 65 on Medicare due to disability and covered through an employer with 100 or more workers, the employer plan is primary. If you’re in a liability, no-fault, or workers’ compensation situation, that coverage always pays first for related care. In every other typical scenario, Medicare is your primary payer.