For many years, a great advantage of turning 65 was that you could go on Medicare, buy a supplemental policy to close any gaps, and leave the private health-insurance market. But now, about 20 percent of Medicare beneficiaries nationwide find themselves right back at square one: poring over confusing HMO rules, being restricted to certain doctors and hospitals, and coping with unanticipated high co-pays and deductibles.

The change began with the 2003 Medicare Modernization Act, which directed special subsidies to insurers to offer a privatized version of Medicare called Medicare Advantage.

Because insurers collect 12 to 19 percent more money per patient than original Medicare spends on someone enrolled in that program, they have found Medicare Advantage to be a profitable business. For people in relatively good health, many Medicare Advantage plans might be a better deal than original Medicare plus a supplemental plan. But consumer advocates warn that the plans are not for everyone.

Advocates have heard from seniors who have signed up for Medicare Advantage plans without realizing that they might restrict choice of doctors and hospitals, void certain retiree health benefits, require preapproval for many treatments, impose co-pays and coinsurance fees that can run up huge bills for people who fall unexpectedly ill, and make them unable to obtain a traditional Medicare supplement plan if they want to switch back to regular Medicare.

If you are considering a Medicare Advantage plan, find out whether it is right for you by seeking counseling. To find a qualified counselor near you, contact your state's Health Insurance Assistance Program.