CMS-40B Medicare Part B Application Form Instructions

CMS-40B is the Social Security Administration form used to enroll in Medicare Part B, which covers outpatient care, doctor services, preventive services, and other medical services. If you delayed Part B because you or your spouse had qualifying job-based coverage, you will usually submit Form CMS-40B with Form CMS-L564 to request a Special Enrollment Period. In 2026, the safest approach is to complete the form carefully and submit it to Social Security, then keep copies of everything you send.

Form CMS-40B does not choose a Medicare Advantage plan, Medigap policy, or Part D drug plan. It is the application to start Part B under Original Medicare. Your next steps can depend on whether you are turning 65, retiring, or replacing employer coverage.

When you should use the CMS-40B form

You generally use CMS-40B when you need to actively enroll in Part B instead of being enrolled automatically. The official Medicare enrollment guidance explains that people who are already receiving Social Security benefits before they become eligible for Medicare are generally enrolled automatically, while many others need to sign up themselves.

• Use it during your Initial Enrollment Period: This is the first chance many people have to enroll around age 65. See the medicare initial enrollment period 7 month window explained guide for timing details.
• Use it after active employer coverage: You may qualify for a Special Enrollment Period when employment or group health plan coverage ends.
• Use it during the General Enrollment Period: This can be an option if you missed an earlier opportunity, although a late enrollment penalty may apply depending on your circumstances.

Medicare enrollment is not the same as the medicare annual enrollment period. The annual period is mainly for changing Medicare health or drug coverage after you already have Medicare.

How to fill out CMS-40B without common mistakes

Start with the current version of Form CMS-40B from the Centers for Medicare & Medicaid Services. The Centers for Medicare & Medicaid Services is the federal agency that administers Medicare. Read the form instructions before choosing an effective date, because the right date depends on your enrollment period and coverage situation.

For a Special Enrollment Period, Form CMS-L564 matters because it documents your employer group health plan coverage. The CMS-L564 form instructions explain that the employer completes the evidence section. If the employer cannot complete the form, Social Security may consider other evidence, but you should ask Social Security what documentation is acceptable for your situation.

Do not assume any employer coverage allows you to delay Part B. Medicare creditable coverage can be important for Part D decisions, but Part B enrollment rules are separate. Coverage through an employer with fewer than 20 employees, retiree coverage, COBRA, and Health Insurance Marketplace coverage can have different effects on your Medicare timing.

What happens after you submit CMS-40B

Social Security reviews your request and determines your Part B effective date. If you are approved, your Medicare record and card information should reflect Part B coverage. Review the effective date before scheduling care or ending other insurance, because you may need to coordinate the transition.

Once Part B begins, you can consider your next coverage decisions. Original Medicare does not include most outpatient prescription drug coverage, and it has cost sharing that some people address with Part D and a Medigap policy, or by choosing a Medicare Advantage plan. These choices have their own deadlines, so review the available medicare enrollment periods rather than assuming every option is available at any time.

If you are retiring, also compare the end date of your workplace plan with the desired Part B start date. Our guide to medicare when you retire can help you map out that handoff.

Get help sorting out your next Medicare step

The CMS-40B form is only one part of the transition to Medicare. Before you submit it, it helps to understand when Part B starts, what other coverage ends, and which Medicare choices you can make afterward. Use the guidance form below to get plain-language help reviewing your enrollment timing and coverage options. Medicare Plan Path is an independent guidance resource and is not affiliated with or endorsed by the U.S. government or Medicare.