BALTIMORE -- The attorney general says some health care providers are illegally billing members of health maintenance organizations directly, but the state hospital association says the bills are legal.

Members of HMOs are stuck in the middle of the dispute between HMOs and medical care providers who want their bills paid promptly.

Many providers are illegally billing the patients directly, says Attorney General J. Joseph Curran Jr. (D).

Curran said Monday that these bills don't have to be paid. The state hospital association contends that Curran is wrong, in at least some cases.

As a result of the dispute, some patients are facing bills for thousands of dollars in expenses they thought would be covered automatically when they joined an HMO. Some who haven't paid have had to deal with dunning letters and other threats.

HMOs typically create networks of doctors, hospitals and others to provide coverage at a set monthly fee. They've long touted the claim that their system is cheaper and virtually free of paperwork for members.

About 850,000 people in Maryland belong to HMOs.

The Health Education and Advocacy Unit of the Attorney General's Office has received 50 to 60 calls this year from HMO members who have received the illegal bills.

"That's really the tip of an iceberg," Curran said.

Some medical providers, including the state's hospital association, have claimed the 1989 state law that supposedly bars them from sending out such bills applies only to health care providers within an HMO's network.

That means a patient referred by his HMO to an independent doctor could risk getting a bill from that doctor that he never anticipated.

However, in April a circuit judge in Carroll County ruled against the University of Maryland Medical Systems on that very claim.

The judge ruled that the law covers all HMO members and all medical providers.