WASHINGTON — The nation's 34 million Medicare beneficiaries are about to have a big burden lifted from their shoulders: They no longer will have to spend hours filling out complex--and often confusing--forms to get reimbursed for their doctor bills.

Starting this month, a new law will require physicians themselves to fill out the claim forms for payments from Medicare.

The federal program will spend $41.9 billion this fiscal year helping to pay medical bills for people over 65 and the disabled of all ages. It also will be limiting physicians' fees for all Medicare patients. Starting in January, doctors will not be permitted to charge patients more than 25% above the recommended fee schedules of Medicare.

But the new program may prove a mixed blessing for some recipients. It gives physicians up to a year to turn in the forms. As a result, patients now may have to wait months--possibly as long as a year--to get their reimbursement checks from Medicare.

Officials at the Health Care Financing Administration are generally confident that most physicians, already experienced in dealing with Medicare paperwork, will file the forms promptly.

But Kathleen Vallee, regional coordinator for the Los Angeles Medicare Advocacy Project, one of 24 programs run by California's Health Insurance Advocacy and Counseling Program, is not as sanguine.

With penalties for late filing unusually light, there's little incentive for physicians to send their forms in very frequently, Vallee said. "A doctor could decide he doesn't want to hire another bookkeeper and could let the bills pile up in the back room," she said.

Still, Vallee concedes, the previous system had its flaws as well.

Some older people, puzzled about what Medicare will cover and intimidated by the forms, have neglected to fill out the papers. Beneficiaries lose between $90 million and $130 million a year on Medicare claims they never file, according to a 1989 government survey.

"We've had hundreds of frustrated people come in with paper bags filled with bills and Medicare forms, and say: 'Please help me,' " Vallee said.

Spokesmen for beneficiaries' groups agree. "Now Medicare beneficiaries will receive the benefits to which they are entitled without having to deal with the burdensome paperwork," said Horace Deets, executive director of the American Assn. of Retired Persons.