CMS Form 40B is the one-page application you send to Social Security to enroll in Medicare Part B when you already have Part A. To fill out CMS-40B, download the current version (revised July 2025) from the CMS website, complete three short sections covering your Medicare number, your group health coverage history, and your signature, and submit it by fax, mail, or in person to your local Social Security office.1Centers for Medicare & Medicaid Services. CMS Form 40B – Request for Enrollment in Medicare Part B Most people filing it are enrolling through a Special Enrollment Period after leaving employer coverage, and those filers also need Form L564.
Confirm You’re in a Valid Enrollment Window First
Social Security will only process Form 40B if you’re inside one of three windows. Filing outside them means the form gets rejected or held.
- Special Enrollment Period: an eight-month window that opens the month your employment ends or the month your group health coverage ends, whichever comes first, if you delayed Part B because you had coverage through your or your spouse’s current employer. This is the only path that avoids a late enrollment penalty after your initial eligibility window closes.2eCFR. 42 CFR 407.20 – Special Enrollment Period Related to Coverage Under Group Health Plans
- General Enrollment Period: January 1 through March 31 each year, for people who missed initial enrollment and don’t qualify for a SEP.3eCFR. 42 CFR 407.15 – General Enrollment Period
- Exceptional circumstances: a natural disaster or emergency that prevented on-time enrollment may open a separate SEP.4Medicare. Special Enrollment Periods
Section 1: Your Identifying Information
Enter your Medicare number (from your red, white, and blue Medicare card), your full legal name, mailing address, phone number, and email address. The Medicare number is the field to double-check. If you have an older card that shows a Social Security number, use the Medicare Beneficiary Identifier from the replacement card you should have received.
Section 2: The Enrollment Questions
This is the substantive part of the form. Four numbered items and a start-date box.
Item 1 asks whether you have or had group health plan coverage through an employer or union since turning 65. Check “Yes” if you’re enrolling through a Special Enrollment Period. A “Yes” here means you also need to complete Item 3.
Item 2 asks whether you served as an international volunteer for a nonprofit that provided health coverage, such as with the Peace Corps. This applies to a narrow group. A “Yes” also sends you to Item 3.
Item 3 is the dates section. Enter start and end dates in month/year format for your employment period, and separately for your health coverage period. Check “Not ended” if either is still active. If your coverage came through your spouse’s employer, put those dates here. Use a separate sheet if you need more room.
Item 4 asks whether an employer, insurer, or other entity required you to enroll in Part B. This sometimes happens when an employer moves workers onto Medicare as primary coverage. If yes, explain in the space provided and include any documentation you have.
Below Item 4 is the coverage start date box for SEP applicants. If you’re enrolling while still on a group health plan, or within the first full month after that coverage ends, you pick one of two options: coverage starting the first day of the month you enroll, or the first day of any of the three months after you enroll. Write in your preferred month and year if you choose the delayed option. This choice lets you line up your Part B start with when your employer coverage actually ends so you’re not paying twice or leaving a gap. It also has tax consequences if you have an HSA (see below).
Section 3: Signature
Sign and date the form. If you sign with a mark (X) rather than a full signature, a witness who knows you must also sign, print their name, and date the form.
Form L564: The Employer Verification for SEP Filers
If you’re using a Special Enrollment Period, submit CMS Form L564 (Request for Employment Information) with your Form 40B. It proves you had group coverage through current employment.5Centers for Medicare & Medicaid Services. CMS-L564 – Request for Employment Information
You complete Section A with your identifying information. Then send or hand it to your employer (or former employer) to fill in Section B, which asks for the dates your group health coverage began and ended and your employment dates. Social Security won’t process a SEP enrollment without it.6Centers for Medicare & Medicaid Services. Medicare Request for Employment Information
If your former employer is slow or out of business, gather substitute proof: COBRA election notices, benefits statements, or pay stubs showing health insurance deductions. Add a short written explanation of why you couldn’t get L564 completed.
How to Submit the Form
Social Security, not CMS, handles Part B enrollment. Send the completed Form 40B (and L564 if applicable) to your local Social Security office by one of three methods:7Social Security Administration. Sign Up for Part B Only
- Fax: the fastest paper option, and you get a transmission confirmation. Find your local fax number through the Social Security Office Locator at secure.ssa.gov/ICON/main.jsp.
- Mail: the same locator gives you the mailing address. Certified mail gets you delivery confirmation.
- In person: bring the completed forms to your local office. Appointments move faster than walk-ins.
There is no online submission portal for Form 40B. Social Security’s Part B enrollment page directs applicants to fax or mail only. Keep copies of everything you send and note the submission date in case you later need to prove your enrollment was timely.
When Your Coverage Starts
If you enrolled through a SEP, coverage begins on the date you selected on the form. If you didn’t specify, Part B typically starts the month after Social Security receives your paperwork.8Medicare. When Can I Sign Up for Medicare
If you enrolled during the General Enrollment Period, coverage starts the first day of the month after you sign up. Enroll in February, coverage begins March 1.9eCFR. 42 CFR 407.25 – Beginning of Entitlement – Individual Enrollment
Once your enrollment is finalized, you’ll get a new Medicare card in the mail showing both Part A and Part B and the effective date. To check enrollment status before the card arrives, call Social Security at 1-800-772-1213.
Choosing a Start Date If You Have an HSA
The start-date box in Section 2 has a tax consequence worth thinking about before you sign. Enrolling in Part B ends your Health Savings Account contribution eligibility as of the coverage start date. Any contributions made after that date are excess contributions and carry a 6 percent excise tax for every year they remain in the account.10Internal Revenue Service. Publication 969 – Health Savings Accounts and Other Tax-Favored Accounts
If you want to make a final HSA contribution, picking a start date one to three months out on Form 40B can preserve your eligibility for those months. You can still spend existing HSA funds tax-free on qualified medical expenses, including Medicare premiums; the restriction is on new contributions.
What Filing Late Costs
If you go without Part B and don’t qualify for a SEP, expect a late enrollment penalty: an extra 10 percent added to your standard premium for every full 12-month period you could have had Part B but didn’t.11Medicare. Avoid Late Enrollment Penalties The penalty is permanent for as long as you have Part B. In 2026, the standard premium is $202.90 with a $283 annual deductible, so filing Form 40B during the right window is the difference between the standard premium and a lifelong surcharge on top of it.12Centers for Medicare & Medicaid Services. 2026 Medicare Parts A and B Premiums and Deductibles