CMS-40B Application Form: How to Apply for Medicare Part B
What Form CMS-40B Is For
Form CMS-40B is the official application to enroll in Medicare Part B (Medical Insurance). It can be used during your Initial Enrollment Period, during a Special Enrollment Period after losing employer coverage, or during the General Enrollment Period (January 1 through March 31) if you missed your initial window.
If you're enrolling in Part B for the first time at 65 alongside Part A, you generally apply through Social Security directly (online, by phone, or in person) without needing CMS-40B specifically. The form becomes essential when you delayed Part B because you had employer coverage and are now transitioning to Medicare.
How to Fill Out the CMS-40B
The form is one page. Here's what each section asks for:
Personal information. Your name, date of birth, sex, and Medicare claim number (the number on your Medicare card, including any letter suffix).
Current coverage information. Whether you're currently enrolled in Part A, whether you currently have Part B, and the reason you're applying. If you're applying during a Special Enrollment Period, check that box. If you're applying during the General Enrollment Period, check that one instead.
Employment and coverage dates. The companion CMS-L564 records when your employer group health plan coverage and active employment started and ended (or "present" if coverage is still active).
The Remarks section. This is the most important underused field on the form. Write: "I want Part B coverage to begin [month/year]." If you don't specify, the SSA defaults to a start date that may leave you with a gap in coverage. Requesting a specific date gives you control over when your Part B premium starts and when your coverage kicks in.
Signature and date. Sign the form. If you're filing on behalf of someone else (a parent, for example), you'll need a power of attorney or other authorization — Form SSA-1696 (Appointment of Representative) is the standard way to establish that authority.
What Else to Submit With the CMS-40B
The CMS-40B alone doesn't prove you qualify for a Special Enrollment Period. You also need:
Form CMS-L564 (Request for Employment Information). This is the companion form where your employer certifies that you had group health coverage based on current active employment. Section A is yours to fill out; Section B goes to your employer's HR department.
Supporting documentation if your employer can't complete the CMS-L564: W-2 forms, pay stubs showing insurance deductions, insurance cards, benefits enrollment letters, or COBRA election notices that document when active employment ended.
Submit both forms together as a package. Sending the CMS-40B without the CMS-L564 will delay processing because the SSA can't verify your SEP eligibility.
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Where and How to Submit
You have several filing options:
- In person at your local Social Security office. Bring the originals plus copies of everything. Ask for a receipt.
- By mail to your local Social Security office. Use certified mail with return receipt so you have proof of the filing date.
- By fax. Call your local SSA office for their fax number.
- Online through your my Social Security account at ssa.gov, where you can upload documents.
Processing time varies. During the General Enrollment Period (January through March), SSA offices handle higher volumes and turnaround is slower. Special Enrollment Period applications filed with complete documentation — CMS-40B plus CMS-L564 plus supporting evidence — typically process faster than incomplete submissions.
When Part B Coverage Starts
Your effective date depends on when and how you enroll:
Special Enrollment Period. Coverage begins the first day of the month after the month SSA processes your enrollment. If you specify a preferred start date in the Remarks section and it's within your SEP window, SSA will generally honor it.
General Enrollment Period. If you missed all other deadlines and enroll between January 1 and March 31, coverage begins the first day of the month after the month you enroll. That means you may have a gap between when you enroll and when coverage begins — and you'll pay a permanent late-enrollment penalty.
This is precisely why acting during your SEP matters so much. The GEP fallback means months without Part B coverage and a lifetime premium surcharge.
Common CMS-40B Mistakes
Not specifying a start date. If you leave the Remarks section blank, SSA picks the default effective date. This can create a gap between your employer coverage ending and Part B beginning, during which you have no primary medical insurance.
Filing without the CMS-L564. The SSA will send you a letter asking for employer verification, adding weeks to the process. Submit both forms together.
Missing the SEP deadline. Your Special Enrollment Period lasts 8 months from the date your employment or employer coverage ended. Filing on month 7 is fine. Filing on month 9 means you've missed the SEP and must wait for the GEP — with penalties.
Using an outdated form. The CMS-40B is periodically updated. Download the current version from the CMS forms page to avoid having your application returned.
The Medicare Late-Enrollment Penalties and Special Enrollment guide includes the complete CMS-L564 filing packet with a secondary evidence checklist and pre-written employer request letters, so you can submit a bulletproof CMS-40B package.
Get Your Free Medicare Late-Enrollment Penalties and Special Enrollment — Quick-Start Checklist
Download the Medicare Late-Enrollment Penalties and Special Enrollment — Quick-Start Checklist — a printable guide with checklists, scripts, and action plans you can start using today.