You are eligible if you already receive benefits under Medicare Part A or Part B, the most common elements of Medicare.

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"By law, Social Security benefits and Medicare Part A are linked together, " explains Kia Green Anderson, a spokeswoman for the Social Security Administration.

WSJ: Collect the Most Social Security

"If a person is entitled to Social Security benefits and meets the Medicare requirements, they are entitled to Medicare Part A, " which covers hospital care and is usually free.

WSJ: Collect the Most Social Security