It depends on how long you have been receiving Social Security disability insurance (SSDI) payments. You are automatically enrolled in Medicare Part A and Part B after you have been receiving SSDI payments for two years. However, people with disabilities who have Amyotrophic Lateral Sclerosis (ALS) do not have to wait two years and are eligible for Medicare immediately upon collecting SSDI, and people with end-stage renal disease (ESRD) are eligible for Medicare three months after beginning dialysis treatment or immediately after receiving a kidney transplant.