When you have other insurance aside from Medicare, there are rules on whether who pays first. If you have a Retiree Insurance , Medicare pays first.  If you are 65 years or older and you have a group plan coverage based on your or your's spouse's current employment and the employer has 20 or more employees, your group health plan pays first. If you're under 65, have a disability and a group health plan coverage based on your or your family member's current employment, and the employer has more than 100 or more employees, your group health plan pays first. If you're under 65 and have a disability and the employer has fewer than 100 employees , Medicare pays first.  If you have Medicare because of End-Stage Renal Disease (ESRD ) , your group health plan will pay first for the first 30 months after you become eligible to enroll in Medicare. Medicare will then pay after this 30-month period.  If you have Medicaid , Medicaid pays first. ...