42 C.F.R. § 406.6 : Public Health — CENTERS FOR MEDICARE & MEDICAID SERVICES, DEPARTMENT OF HEALTH AND HUMAN SERVICES — MEDICARE PROGRAM — HOSPITAL INSURANCE ELIGIBILITY AND ENTITLEMENT — GENERAL PROVISIONS — Application or enrollment for hospital insurance

(a) Basic provision. In most cases, eligibility for Medicare Part A is a result of entitlement to monthly social security or railroad retirement cash benefits or eligibility for monthly social security cash benefits. This section specifies the individuals who need not file an application to become entitled to hospital insurance, those who must file an application, and those who must enroll.
(b) Individuals who need not file an application for hospital insurance. An individual who meets any of the following conditions need not file an application for hospital insurance:
(1) Is under age 65 and has been entitled, for more than 24 months, to monthly social security or railroad retirement benefits based on disability.
(2) At the time of attainment of age 65, is entitled to monthly social security or railroad retirement benefits.
(3) Establishes entitlement to monthly social security or railroad retirement benefits at any time after attaining age 65.
(c) Individuals who must file an application for hospital insurance. An individual must file an application for hospital insurance if he or she seeks entitlement to hospital insurance on the basis of-
(1) The transitional provisions set forth in §406.11;
(2) Deemed entitlement to disabled widow's or widower's benefit under certain circumstances as provided in §406.12;
(3) A diagnosis of end-stage renal disease, as specified in §406.13;
(4) Effective January 1, 1981, eligibility for social security cash benefits, as specified in §406.10(a)(3), if the individual has attained age 65 without applying for those benefits; or
(5) The special provisions applicable to government employment as set forth in §406.15.
(d) When application is deemed to be filed.
(1) An application based on the transitional provisions or on ESRD is deemed to be filed in the first month of eligibility if it is filed not more than 3 months before the first month, and is retroactive to that month if filed within 12 months after the first month. An application filed more than 12 months after the first month of eligibility is retroactive to the 12th month before the month it is filed.
(2) An application for deemed entitlement to disabled widow's or widower's benefits, that is filed before the first month in which the individual meets all conditions of entitlement for this benefit, will be deemed a valid application if those conditions are met before an initial determination, reconsideration, or hearing decision is made on the application. If the conditions are met after the date of any hearing decision, a new application will have to be filed. An application validly filed within 12 months after the first month of eligibility is retroactive to that first month. If filed more than 12 months after that first month, it is retroactive to the 12th month before the month of filing.
(3) Effective June 8, 1980, an application based on eligibility for social security benefits at or after age 65, that is filed before the first month in which the individual meets all eligibility conditions for this benefit, will be deemed a valid application if those conditions are met before an initial determination, reconsideration, or hearing decision is made on the application. If the conditions are met after the date of any hearing decision, a new application will have to be filed.
(4) Effective March 1, 1981, an application under §406.10 that is validly filed within 6 months after the first month of eligibility is retroactive to that first month. If filed more than 6 months after that first month, it is retroactive to the 6th month before the month of filing.
(e) Individuals who must enroll for hospital insurance. An individual who must pay a monthly premium for hospital insurance must enroll in accordance with the procedures set forth in §406.21.

42 C.F.R. §406.6

48 FR 12536, 3/25/1983, as amended at 50 FR 33033, 8/16/1985; 53 FR 47202, 11/22/1988; 61 FR 40345, 8/2/1996