If you’ve received Social Security retirement benefits for at least four months before your 65th birthday, you’ll be enrolled automatically in Parts A and B of Medicare at the beginning of the month you turn 65. If your birthday is on the first day of a month, your coverage will start a month earlier.

If you live in the U.S. territory of Puerto Rico, the rules are slightly different. But we’ll come back to that in a minute.

Three months before your Medicare coverage begins, you’ll receive a Welcome to Medicare package in your mailbox. It will include a Medicare card, similar to the one above, a letter and a booklet that explains the decisions you’ll need to make before your coverage begins.
 

You’ll also get a letter reminding you of your benefits about a month before they begin.
 

Unless you or your spouse have paid less than 40 calendar quarters of Medicare taxes (generally taken automatically from most workers’ paychecks), Part A will be free. And Part B premiums will be deducted automatically from your monthly Social Security benefits.

If your benefits aren’t enough to cover the premium or you stop getting Social Security benefits, you’ll get a bill for your Part B premiums every three months. If you’d prefer to pay your bill automatically, you can sign up for Medicare Easy Pay. Financial assistance is also available if you need help paying your Part B premiums and other Medicare costs.

If you have health insurance from a company with fewer than 20 employees, it’s important to keep Part B to avoid gaps in coverage and a late enrollment penalty.

If you have health insurance from a company with 20 or more employees, you may want to delay enrolling in Part B because you would have to pay for it, and you'll be eligible for a special enrollment period after you lose your employer-based coverage. (You can’t delay your Part A coverage if you’re receiving Social Security benefits and don’t have to pay a premium for Part A.)

Parts A and B, even if you will be enrolled automatically, won’t cover all your health care costs. If you decide on original Medicare coverage, you’ll still need to enroll in a Part D prescription drug plan if you don’t have coverage through a spouse or from a previous employer and may need to choose a Medigap supplemental insurance plan. You also may want to investigate low-cost dental, hearing and vision plans.

If Medicare Advantage plans intrigue you, you’ll need to sign up for one separately. Sometimes you’ll have to pay extra to participate and you’ll have additional cost sharing associated with services you use in those plans.

If you don’t want Part B, return your Medicare card. You’ll get another one. Here’s what you need to do.
 

1. Write down your Medicare number so you can use it if you need to go to the hospital or get other Part A services before your new card arrives.
 

2. Check the box after “I do NOT want Part B” on the back of your Medicare card.
 

3. Sign the back of your card.
 

4. Follow the instructions and return the card before your coverage is supposed to start.
 

Medicare will send you a new card in a few weeks that shows you have only Part A.
 

If you decide to delay signing up for Part B, you’ll need to enroll no later than eight months after you lose your employer-based coverage, to avoid a late enrollment penalty; this is called the special enrollment period.
 

If you live in Puerto Rico and are receiving Social Security benefits, you’ll be enrolled automatically in Part A and will receive a Welcome to Medicare package in the mail. But you’ll have to take steps to sign up for Part B.
 

U.S. territories were included in Medicare from the beginning of the program in 1965; however, changes in the law in the 1980s excluded Social Security recipients who live in Puerto Rico from automatic enrollment in Part B. About 9 in 10 residents of U.S. territories live in Puerto Rico.
 

So you’ll have to take steps to sign up for Part B if you live there. You usually have to sign up during your seven-month initial enrollment period, three months before the month you turn 65 to three months afterward. If you miss this deadline, you may face gaps in coverage and have to pay a late enrollment penalty. Like in the states, you can delay enrollment if you have coverage from an employer with 20 or more employees.
 

To sign up for Part B during your initial enrollment period, fill out Form CMS-40B and mail it to your local Social Security office. Or you can call the Social Security Administration at 800-772-1213 and request the form and a preaddressed return envelope. It’s also possible to enroll at your local Social Security office.
 

For more information in English and Spanish, see the Social Security Administration’s Medicare in Puerto Rico fact sheet.