CMS-40B is the form you use to add Part B when you already have Part A. Its current title is "Request for Enrollment in Medicare Part B (Medical Insurance)." The edition is CMS-40B (07/2025), OMB No. 0938-1230, expiring 07/31/2028. Older Social Security pages call it the "Application for Enrollment in Medicare Part B." It's the same form.
The form can be used in your Initial Enrollment Period, the General Enrollment Period (January 1 to March 31), or a Special Enrollment Period. If you're signing up because your job-based coverage is ending, you send it together with CMS-L564.
First, check that you have Part A
The form says: "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 manual adds that only people with premium-free Part A can use CMS-40B. People who aren't eligible for premium-free Part A use form CMS-4040 to enroll in Part B only (POMS HI 00805.075).
If you never signed up for Part A, Social Security lets people 65 and older apply online for Parts A and B at ssa.gov/medicare/sign-up. Your CMS-L564 is still your proof of job-based coverage. If you contribute to a Health Savings Account, read the HSA warning in working past 65 at a small employer before you apply for Part A.
Section 1: Basic information
The form says all fields are required unless noted.
- Medicare Number. It's on your Medicare card. Write it in by hand. Our kit leaves this box blank on purpose, because we never ask for it.
- Name. First, middle, last and suffix.
- Mailing address. Number and street, P.O. Box or route, then city, state and ZIP code.
- Phone number.
- Email address.
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Section 2: Enrollment in Medicare Part B
Question 1: Job-based coverage since 65
"Do you have (or did you have) coverage through an employer or union group health plan since you turned 65?" If you're using the Special Enrollment Period, the answer is Yes. A Yes means you also complete item 3.
Right under this question, the form reminds you that once you have Part B, you pay premiums for every month you have it.
Question 2: International volunteer
This asks whether you are (or were) an international volunteer for a non-profit that gave you health coverage. Most people answer No. If you answer Yes, you complete item 3 for that too.
Question 3: Your dates
Enter every date as month and year (mm/yyyy). There are three rows, each with a start date, an end date and a "Not ended" box:
- Dates you (or your spouse) worked for an employer that provided health coverage. If your coverage is through your spouse's job, these are your spouse's work dates.
- Dates you worked as a volunteer outside the U.S. Skip it unless you answered Yes to question 2.
- Dates of health coverage from employer (or non-profit organization). When your coverage started, and when it ended or "Not ended."
The form says to attach a separate sheet if you need more space, for example if you had coverage through more than one employer since 65. It also says to have your employer fill out CMS-L564 and return it with your application. Your dates here should match Section B of the L564.
Question 4: Did someone ask you to enroll?
"Has an employer, health insurance provider, or other entity asked or required you to enroll in Part B?" If Yes, explain how and why in the space provided, and include proof or documentation with the form.
Choosing your coverage start date
This box is for people enrolling while they're still covered by a job-based plan from current work, or during the first full month after that coverage ends. You choose one:
- the first day of the month you enroll, or
- the first day of any of the 3 months after you enroll (write the month and year).
If you enroll later in your 8-month window, this choice doesn't apply. Part B starts the first day of the month after you enroll (POMS HI 00805.276).
Example: your job coverage ends March 31, 2027. You send CMS-40B in February 2027. You can pick February, March, April or May 2027. Picking April 2027, the first month without job coverage, avoids a gap. It also avoids paying Part B premiums for months your job plan still covers you.
Medicare.gov describes this a little differently
Medicare.gov says coverage generally starts the month after you sign up, and that you can ask to delay it up to 3 months. The 07/2025 form and Social Security's manual also allow the month you enroll. Mark your choice in the box so there's no guessing. More in when does Part B start.
Section 3: Signatures
- Signature of applicant
- Date signed (mm/dd/yyyy)
If you sign with a mark (X), a witness who knows you must also sign. The witness gives their first and last name, signature and the date. The rules require the request to be signed by you or someone acting for you (42 CFR 407.22).
Common mistakes
- Using CMS-40B when you don't have Part A yet.
- Answering Yes to question 1 but leaving item 3 blank.
- Dates that don't match the employer's Section B on the L564.
- Choosing a start month more than 3 months after the month you send the form.
- Mixing up date formats: mm/yyyy for work and coverage, mm/dd/yyyy for signatures.
- Forgetting to sign, or sending it after your window has closed.
Where to send it
Mail or fax the signed form to your local Social Security office, which you can find at SSA.gov/locator. CMS says you can also take it to the office in person. During the Special Enrollment Period you can apply online instead. See how to submit CMS-40B and CMS-L564.