Updated October 18, 2026. Quick answer: Form CMS-40B is the Request for Enrollment in Medicare Part B. It is for people who already have Medicare Part A and want to add Part B, whether in their Initial Enrollment Period, the General Enrollment Period or a Special Enrollment Period. People with employer coverage from current work answer extra questions and send a second form, CMS-L564, with it. You mail or fax it to your local Social Security office.
Quick reference
| Item | As printed on the form or stated in Social Security’s Medicare booklet |
|---|---|
| What the form is | A request to enroll in Medicare Part B (Medical Insurance) |
| Who uses it | People who already have Medicare Part A |
| Employer coverage | Items 1 to 4 in Section 2, plus form CMS-L564 |
| Initial Enrollment Period | 7 months: 3 months before the month you turn 65, the month itself and 3 months after (age 65) |
| Employer-coverage window | 8 months after the employment or the group coverage ends |
| Late enrollment premium | 10% more for each 12 months you were eligible and did not sign up |
| Where it goes | Your local Social Security office, by mail or fax |
| Help | 1-800-772-1213 (TTY 1-800-325-0778) |
Who should use Form CMS-40B
The form opens with its purpose: “Use this form if you already have Medicare Part A and want to sign up for Part B (Medical Insurance).” It lists three windows for signing up: your Initial Enrollment Period, the General Enrollment Period each year, and a Special Enrollment Period if you qualify for one.
It is not the form for first-time Medicare enrollment. The instructions say “If you don’t have Part A, don’t complete this application. Contact Social Security to apply for Medicare for the first time.”
Social Security’s Medicare booklet describes the first window like this: “You can do so during the seven-month period that begins three months before the month you turn 65. This period includes the month you turn 65 and ends three months after the month you turn 65.”
What the form asks
Section 1 collects your Medicare number, name, mailing address, phone number and email address. Section 2 is where employer coverage comes in. Item 1 asks whether you have, or had, “coverage through an employer or union group health plan since you turned 65” and item 2 asks about international volunteer work that came with health coverage. A yes to either means you complete item 3, which asks for your employment and coverage dates.
Item 3 also tells you what to attach: “Have your employer fill out the form CMS-L564 (Request for Employment Information) and return it with your application.” Our page on Form CMS-L564 walks through that second form. Item 4 asks whether “an employer, health insurance provider, or other entity asked or required you to enroll in Part B” and wants an explanation and proof if the answer is yes.
The form also warns about cost: “Note: If you sign up for Part B, you must pay premiums for every month you have the coverage.”
Choosing your coverage start date
A block on the form lets some people pick the start month. It says “If you’re enrolling in Medicare while you’re still covered by a group health plan based on current employment (or during the first full month you’re not enrolled in the group health plan), you can choose when your Medicare coverage will start.” The choices are the first day of the month you enroll, or the first day of any of the next three months, written as month and year.
The Social Security booklet gives the matching rule for the employer-coverage window: “Enroll in Medicare Part B during the eight-month period that begins the month after the employment ends or the group health coverage ends, whichever happens first.” It adds “You can’t enroll using an SEP until your IEP is over.”
Timing, penalties and what does not count
The booklet states the cost of missing the first window: “Your monthly premium will go up 10% for each 12-month period you were eligible for Part B, but didn’t sign up for it.” If you also miss an employer-coverage window, it says “If you don’t enroll by the end of the eight-month period, you’ll have to wait until the next GEP, which begins January 1 of the next year.”
Not every coverage counts as employer coverage. The booklet says “COBRA and retiree health coverage don’t count as current employer coverage.” For the rules that apply to you, see working past 65 and Medicare and the Part B penalty calculator.
Where to send it
The form says “Mail or fax your completed, signed form to your local Social Security office.” The booklet adds that people using the employer-coverage window can “Fax your CMS-40B and employer-signed CMS-L564 forms to your local Social Security office.” Keep a copy of everything you send.
The edition read for this page is Form CMS-40B (07/2025) as the agency served it on 2026-10-18. Check the edition printed on the copy you send, because a later edition can change the questions, the mailing instructions and the evidence asked for.
Common mistakes
- Using this form before you have Part A. The form says to contact Social Security to apply for Medicare for the first time instead.
- Answering yes to the employer-coverage items and sending the form without CMS-L564, which the form asks your employer to complete and return with the application.
- Treating COBRA or retiree coverage as current-employment coverage. The booklet says neither counts.
- Waiting past the end of the employer-coverage window, which sends you to the January to March General Enrollment Period and can add the 10% premium increase for each 12-month period.
Related: Form CMS-L564, Form CMS-1763, the Medicare enrollment window calculator, and Social Security and Medicare forms.
Sources
- Form CMS-40B (07/2025), Request for Enrollment in Medicare Part B (Medical Insurance): https://www.cms.gov/Medicare/CMS-Forms/CMS-Forms/downloads/cms40b-e.pdf
- SSA Publication 05-10043 (2026), Medicare: https://www.ssa.gov/pubs/EN-05-10043.pdf
Read from the Social Security Administration and Centers for Medicare and Medicaid Services documents linked above on October 18, 2026. General information, not legal or benefits advice. Your own facts decide the outcome, and Social Security at 1-800-772-1213 is the right place to confirm anything consequential.