Most of the 6.7 million elderly Americans who belong to HMOs are going to be charged more for their medical care, and at least 250,000 such patients will be dropped outright by their health plans, health industry officials said yesterday.

As a result of the cutbacks, about two-thirds of the Medicare patients in managed-care plans will face higher costs for prescription drugs at the very time that the Clinton administration and Congress have become eager to ensure that all older Americans are able to afford the medicine they need.

The changes announced yesterday guarantee the second consecutive year of turbulence for the growing ranks of people on Medicare who have chosen managed care, consumer groups, government officials, and industry representatives all agreed.

But why the disruptions are happening is in sharp dispute. Officials of the main managed-care trade organization said plans had no choice but to drop out or cut back because the government is too stingy in paying them. Patient advocates, however, contend that HMOs have promised benefits they could not afford in their zeal to attract patients.

The new round of turmoil is the latest part of an uneasy dance between the managed-care industry and the federal government that was set in motion two years ago, when Congress began trying to encourage more elderly Americans to sign up for managed care -- but also changed its rates for HMOs in a way the industry dislikes.

Yesterday was the deadline for plans to notify the federal agency that runs Medicare whether they would accept elderly patients for the coming year and what services they will offer. The news of more defections comes just eight months after nearly 100 health plans announced that were dropping out of certain communities, stranding some 450,000 patients and provoking an outcry on Capitol Hill.

While Medicare officials said they did not have a complete tally yesterday, the trade group, the American Association of Health Plans, compiled this year's trends through a survey this week of 23 large HMOs that have about 4 million Medicare patients, about two-thirds of all beneficiaries in managed care.

Regionally and elsewhere in the country, the number of patients to be dropped by their HMOs by January appears likely to be somewhat smaller than in the first round and largely concentrated in rural areas. Following a major exodus of Medicare HMOs from the mid-Atlantic last year, a few more plans have announced they intend to pull back. Among them, Sentara Health System has decided to eliminate its Medicare HMO, stranding 14,000 in southeastern Virginia, while officials of Medi-CareFirst HMO said yesterday they will stop serving another 14,000 in southern and western Maryland and on the Eastern Shore.

But in a widespread new pattern, health plans are beginning to charge monthly premiums and higher co-payments for specific services, including hospital stays. Until now, 70 percent of Medicare patients have not had to pay monthly premiums, but now at least 1.5 million people will face new or higher premiums -- often of at least $20 a month, the industry survey found.

Kaiser Permanente Health Care Program is raising premiums for many Medicare patients, but a spokesman said its decision to withdraw from parts of rural Georgia was unrelated to federal reimbursements.

In the Washington area, Medi-CareFirst will begin to charge its 34,000 patients $50 a month.

HMOs are seeking to use new burdens on patients as ammunition in their political fight to try to persuade Congress to pay them more money. Karen Ignagni, president of the American Association of Health Plans, said the program "is definitely in free fall."

But Medicare chief Nancy-Ann Min DeParle said, "We believe plans are being paid adequately to provide care for Medicare beneficiaries." Federal officials noted that a recent study by the General Accounting Office, disputed by industry officials, concluded that Medicare still is paying health plans too much.

"We're disappointed that HMOs are making decisions that will force some Medicare beneficiaries to change their health coverage and at the same time scaring them about their . . . benefits," DeParle said.

Consumers advocates were more pointed. "It has all of the feeling of bait and switch," said Ron Pollack, executive director of a consumer health lobby, Families USA. "You bait people into a plan based on promising the sky, even if that promise should have been known to the HMOs as unrealistic."

Joe Baker, associate director of the New York-based Medicare Rights Center, said the new turbulence suggested that President Clinton was right this week in proposing ways to strengthen the original "fee-for-service" version of Medicare, by adding a prescription drug benefit and starting to cover all costs of certain preventive medical care. "This private marketplace may not stay the course," Baker said.

Some on Capitol Hill agreed. "We cannot rely on private insurers to meet the health needs of our seniors," said Rep. Benjamin L. Cardin (D-Md.), calling on Congress to adopt the changes to Medicare proposed by the White House this week.