EARLY this year, Barbara Plumb, a freelance editor and writer in New York who is on Medicare, received a disturbing letter. Her gynecologist informed her that she was opting out of Medicare. When Ms. Plumb asked her primary-care doctor to recommend another gynecologist who took Medicare, the doctor responded that she didn’t know any  and that if Ms. Plumb found one she liked, could she call and tell her the name?

Many people, just as they become eligible for Medicare, discover that the insurance rug has been pulled out from under them. Some doctors  often internists but also gastroenterologists, gynecologists, psychiatrists and other specialists  are no longer accepting Medicare, either because they have opted out of the insurance system or they are not accepting new patients with Medicare coverage. The doctors’ reasons: reimbursement rates are too low and paperwork too much of a hassle.

When shopping for a doctor, ask if he or she is enrolled with Medicare. If the answer is no, that doctor has opted out of the system. Those who are enrolled fall into two categories, participating and nonparticipating. The latter receive a lower reimbursement from Medicare, and the patient has to pick up more of the bill.

Doctors who have opted out of Medicare can charge whatever they want, but they cannot bill Medicare for reimbursement, nor may their patients. Medigap, or supplemental insurance, policies usually do not provide coverage when Medicare doesn’t, so the entire bill is the patient’s responsibility.