CMS-L564: proving you had employer coverage
If you stayed on an employer’s health plan past 65 and delayed Medicare Part B, this is the form that proves it. You fill in half of it; your employer fills in the rest — and that second half is where this goes wrong.
Checked against CMS-L564 (Revised 03/2025, OMB 0938-0787, expires 03/2028) and CMS-40B (07/2025, OMB 0938-1230). Download both from CMS or SSA, not a third-party form site.
The 8-month rule, from the form itself
“To enroll in Medicare through a Special Enrollment Period, you must have had group health plan coverage through your or your spouse's current employment since the first month you were eligible for Medicare Part B. Your coverage must not have ended more than 8 months ago.”
Two conditions, and both matter. The coverage has to have been continuous since the first month you were eligible for Part B — a gap breaks it. And the eight months run from when the coverage ended, not from when you noticed.
If you qualify for Medicare because of a disability, you must have large group health plan coverage based on your, your spouse's or a family member's current employment.
Who fills in what
CMS estimates the form takes 15 minutes. That is the typing. Getting Section B signed is the real task — and if the employer has been acquired, has outsourced its benefits administration, or no longer exists, it can take weeks. The eight-month clock does not pause while you look, which is why Section B is the part with a lead time and Section A is not.
Three rules on the forms that are easy to miss
You can choose when Part B starts. 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 options are: The first day of the month you enroll, or The first day of any of the 3 months after you enroll. This is on CMS-40B and is easy to miss. It lets you line up the Medicare start date with the end of employer coverage, instead of paying two premiums for overlapping months.
Hours Bank Arrangements have their own section. The form carries a section for Hours Bank Arrangements — common in union and trade plans, where banked hours keep coverage running after work stops. If that is your situation, the date your reserve hours run out matters as much as your last day of work.
There is a question about being pushed onto Medicare. CMS-40B asks whether an employer, insurer or other entity asked or required you to enrol in Part B, and tells you to explain and attach proof if so. Worth answering carefully: it exists because pressuring someone off a group plan and onto Medicare is not always permitted.
CMS-40B also asks whether you were an international volunteer for a non-profit that provided your health coverage — a separate route to a Special Enrollment Period that almost nothing written about this form mentions.
Go by the form. Every timing rule here is quoted from CMS-L564 or CMS-40B with the page it appears on, but CMS revises these forms and we may not have caught the latest one. A wrong enrollment date can mean a premium you pay for life, so check the rule against the PDF you are filing — and tell us if they differ.
Sending them in
“Mail or fax this completed form together with your Application for Enrollment in Medicare (CMS-40B) to your local Social Security office.” Find yours with the SSA office locator. Neither form offers an online submission route. Mail or fax only.
The two forms travel together: CMS-40B is the actual Part B application, CMS-L564 is the evidence that you may apply late. On the application itself: “If you don't have Part A, don't complete this application. Contact Social Security to apply for Medicare for the first time.”
CMS-40B covers three enrolment windows: Initial Enrollment Period; General Enrollment Period from January 1–March 31 each year; If you're eligible for a Special Enrollment Period.
Both forms point to the State Health Insurance Assistance Program for free, personalised and unbiased counselling — the forms' own word is 'unbiased', which is more than can be said for most help offered on this subject. Find your local counsellor at shiphelp.org, or call Social Security on 1-800-772-1213 (TTY 1-800-325-0778). Call the same number and press 2 for service in Spanish.
The penalty this form prevents
The rule is not on CMS-L564 or CMS-40B — which is why this page said nothing about it for a while. It is in the statute. 42 U.S.C. § 1395r(b):
“the monthly premium determined under subsection (a) (without regard to any adjustment under subsection (i)) shall be increased by 10 percent of the monthly premium so determined for each full 12 months (in the same continuous period of eligibility) in which he could have been but was not enrolled.”
10% for each full 12 months, so it steps rather than accrues: eleven uncovered months is no penalty, twenty-three is one step, twenty-four is two. On the 2026 standard premium of $202.90, one step is $20.29 a month and three years late is 30%, or $60.87.
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Months are counted to the close of the enrollment period in which you enrolled, and months covered by a group health plan through current employment are excluded — which is what CMS-L564 demonstrates. Enter only the months that are genuinely uncovered.
The statute computes the increase on the premium “without regard to any adjustment under subsection (i)” — and subsection (i) of the same section is “Reduction in premium subsidy based on income”, which is IRMAA. So the penalty is a percentage of the standard premium even for someone paying the top surcharge. Most pages say the penalty is a percentage of “the standard premium” without saying why. This is why. The two surcharges stack on the same bill without compounding — see the 2026 IRMAA table.
Which months count. Counted to the close of the enrollment period in which the person actually enrolled, not to the date they enrolled. And the exclusion is what this form exists for: “there shall not be taken into account months for which the individual can demonstrate that the individual was enrolled in a group health plan described in section 1395y(b)(1)(A)(v) of this title by reason of the individual's (or the individual's spouse's) current employment status or months during which the individual has not attained the age of 65 and for which the individual can demonstrate that the individual was enrolled in a large group health plan” — The burden is on the individual to demonstrate those months. That is what CMS-L564 is for: it is the employer's signed statement of exactly this.
What the statute does not say. The statute does not state a rounding rule for the resulting amount, and does not say the penalty is permanent in so many words - it states an increase to the monthly premium. Neither CMS-L564 nor CMS-40B states a rounding rule either. We therefore publish the percentage, not a rounded dollar figure. The Part D penalty is a different calculation with a different base and a published rounding rule.
How this relates to IRMAA
Different problem, same premium. IRMAA is a surcharge based on your income two years ago; the late enrolment penalty is a surcharge based on when you signed up. Both are added to the same Part B bill, and someone who worked past 65 on a good salary can end up paying both.
If that is you, the 2026 IRMAA brackets show the income side, how IRMAA works explains why the year that matters is already behind you, and what each threshold costs shows whether it is worth managing.