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How Do You Sign Up for Medicare Part B Only?

Find out how to sign up for Medicare Part B only, when the special enrollment period applies, and which forms you may need.

Direct answer

To sign up for Medicare Part B only, use SSA's Part-B-only process if you already have Part A and previously declined or never enrolled in Part B. If active employer group health plan coverage ended recently, you may qualify for an 8-month Special Enrollment Period without a late penalty. Other timing situations can require a general enrollment or special-situation path.

existing Part A + Part B timing reason + employer coverage status = Part-B-only enrollment path

Part B timing

Already have Part A

Use Part-B-only process

Confirm you are not applying for full Medicare again

Employer coverage ended within 8 months

Use Special Enrollment Period

Employer proof may be needed

Still actively covered by employer plan

Evaluate whether to delay Part B

Retiree or COBRA coverage may not work the same

Missed earlier windows

Use General Enrollment Period or special exception if eligible

Penalties and delayed coverage may apply

Fax or mail application

Use CMS-40B and possibly CMS-L564

Incomplete forms can delay enrollment

Situation	What to do	Risk
Already have Part A	Use Part-B-only process	Confirm you are not applying for full Medicare again
Employer coverage ended within 8 months	Use Special Enrollment Period	Employer proof may be needed
Still actively covered by employer plan	Evaluate whether to delay Part B	Retiree or COBRA coverage may not work the same
Missed earlier windows	Use General Enrollment Period or special exception if eligible	Penalties and delayed coverage may apply
Fax or mail application	Use CMS-40B and possibly CMS-L564	Incomplete forms can delay enrollment

Do not treat every Part B case as online-only

SSA allows online Part-B-only enrollment for some people ending active employer group coverage, but fax, mail, phone, or special-case handling may be needed in other situations.

Add Part B

1Confirm you already have Medicare Part A.
2Identify why you need Part B now and whether active employer coverage recently ended.
3Check whether you are inside a Special Enrollment Period.
4Apply online when your situation fits, or prepare CMS-40B and CMS-L564 for fax or mail.
5Track the effective date so you avoid a coverage gap.

FAQ

Can I sign up for Part B if I already have Part A?

Yes. SSA has a Part-B-only process for people who already have Part A and previously declined or never signed up for Part B.

How long do I have after employer coverage ends?

SSA says people covered by an active employer group health plan since turning 65 may have an 8-month Special Enrollment Period after work or coverage ends.

Which forms are used for Part B by fax or mail?

SSA points to CMS-40B for Part B enrollment and CMS-L564 for employer coverage information when applying during the Special Enrollment Period.

Sources & method

We reviewed these references while writing this answer. Figures are estimates — confirm safety-critical work with a professional. Last updated June 7, 2026.