You've written that your Medicare enrollment deadline depends on whether your employer has fewer than 20 employees. Are these full-time employees with benefits or 20 employees total? I work for an employer with mostly part-time employees without benefits. Do these rules apply to us?

Forget the 20 employees for a moment. The basic question is whether your primary insurer, after age 65, will be Medicare or the workplace plan sponsored by your current employer or your spouse's current employer.

In general, if you're covered by a small workplace plan, the answer is Medicare. After you're 65, the workplace plan stops paying for services that Medicare covers -- so without Medicare, you're really uninsured. Your Medicare enrollment window opens three months before your 65th birthday and closes three months afterward.

By contrast, if you're in a bigger workplace plan, that plan is usually your primary insurer even after you're 65. Your Medicare sign-up deadline is eight months after you leave the job or lose coverage in the plan, whichever comes first.

In either case, missing your deadline means a temporary loss of health insurance -- your next enrollment opportunity may be months later -- and a permanently higher Medicare premium.

So what about the 20 employees?

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If a company employs 20 or more full-time and/or part-time employees for each working day in each of 20 or more calendar weeks in the current or preceding year, its group plan usually -- but not always -- must provide primary coverage to plan participants over age 65.

THE BOTTOM LINE Don't try to figure out your Medicare enrollment deadline by counting your co-workers! Before turning 65, ask both your employer and group insurer who'll be your primary insurer after you're 65 -- and double-check by calling Medicare (800-633-4227) to confirm what they say.

WEBSITES MORE INFORMATION nwsdy.li/medicare and nwsdy.li/healthbenefits

TO ASK THE EXPERT Send questions to Ask the Expert/Act 2, Newsday Newsroom, 235 Pinelawn Rd., Melville, NY 11747-4226, or email act2@newsday.com. Include your name, address and phone number. Questions can be answered only in this column. Advice is offered as general guidance. Check with your own advisers for your specific needs.