What this article assumes
You turned 65 while still covered by an employer group health plan through your own or your spouse's active employment, and you delayed Part B without penalty. Now you're retiring, or the coverage is ending. This is the paperwork side of that transition — the *rules* behind it are covered in working past 65 and Medicare enrollment.
The window is a Special Enrollment Period: eight months from the month the employment or the group coverage ends, whichever comes first. Miss it and you're waiting for the General Enrollment Period (January 1 to March 31) with a Part B late penalty of 10% for each full 12-month period you could have been enrolled — permanently.
Form CMS-40B: your application
This is the Application for Enrollment in Medicare Part B. It's short — name, Medicare number, and a remarks box. In that remarks box, write the date you want Part B to start, in the format the instructions ask for. If you leave it blank, Social Security picks a date, and it may not be the one you wanted.
You already have Part A, so this form is only adding Part B. If you don't have Part A either, that's a different application and worth a call to Social Security first.
Form CMS-L564: the one your employer fills in
This is the Request for Employment Information, and it's the piece people get stuck on. Section A you complete; Section B has to be completed and signed by the employer's HR or benefits department, confirming that you had group health coverage based on active employment and the dates it ran.
Start this early. HR departments are slow, former employers are slower, and a company that has been acquired or closed is slower still. If you genuinely cannot get Section B completed, Social Security can accept alternative proof — pay stubs showing health insurance premiums, W-2s, or written statements from the employer — but you'll need to explain the situation and it takes longer.
Filing them
Both forms go to Social Security, not to Medicare. You can upload them through the online application at [ssa.gov/medicare](https://www.ssa.gov/medicare), fax them, mail them, or take them to a local office. Whatever route you pick, keep a copy of everything you send and note the date.
You'll receive a notice confirming your Part B start date. If nothing arrives in a few weeks, call 1-800-772-1213 and check the status rather than assuming it went through — see how to confirm your coverage actually started.
What comes next, and the deadline nobody mentions
Once Part B is active you'll want drug coverage and something to handle the cost-sharing Original Medicare leaves. That means either a Medicare Advantage plan or a Medigap policy plus a standalone Part D plan.
Here is the deadline that catches people: your six-month Medigap open enrollment window starts the month you are both 65 or older and enrolled in Part B. During it, no Medigap insurer can decline you or charge you more for your health history. After it, in Wyoming and Utah, they can. If Medigap is your plan, do not let that window pass while you think about it. The 2026 standard Part B premium, for budgeting, is $202.90 a month with a $283 annual deductible.
Frequently Asked Questions
How long do I have to enroll in Part B after leaving my job?
Eight months from the month your employment or your group health coverage ends, whichever happens first. This is the employment-based Special Enrollment Period.
What if my old employer won't complete form CMS-L564?
Social Security can accept alternative proof such as pay stubs showing health insurance deductions, W-2 forms, or a written statement from the employer. Explain the situation when you file — it works, it just takes longer.
Does COBRA count as employer coverage for delaying Part B?
No. COBRA is not coverage based on active employment, so it does not extend your Special Enrollment Period. Relying on COBRA past 65 is a common and expensive mistake.
When does my Medigap open enrollment window start?
The first month you are both 65 or older and enrolled in Part B. It runs six months, and during it an insurer cannot turn you down or price you up for health history.
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