Who Is Primary On Medicare? 
If you only have Medicare A & B or Parts A & B plus a Medigap plan , Medicare is going to pay first . When you have more than one plan paying a claim , the process has to follow the coordination of benefits rules to determine who pays first and who pays second . The majority of the time , people who are on Medicare will have two types of coverage when they or their spouse works for an employer . Self-employed individuals will generally only be on Medicare and drop their under 65 policy -LRB- unless their spouse has group coverage they can be on -RRB- . How big the company is and whether or not the insured employee is working will generally determine who pays first . If the employer has 20 or more employees , the employer plan will pay first and Medicare second . If the employer has less than 20 employees , Medicare will pay first and the employer will pay second . If you 're on Medicare for a disability , the employer plan pays first and Medicare pays second if there are 100 or more employees - if there are less than 100 employees , Medicare pays first and the employer plan pays second . 