The compulsory portion of Medicare that is known as the Hospital Insurance Program. All persons 65 years and older who are entitled to benefits under the Old Age, Survivors, Disability and Health Insurance Program or railroad retirement, persons under the age of 65 who have been eligible for disability for more than two years, and insured workers (and their dependents) requiring renal dialysis or kidney transplantation are automatically enrolled in Medicare Part A.

Synonyms: Hospital Insurance Program, Medicare, Medicare Part A, Medicare Hospital Insurance Program, Part A, Medicare