The IPA -- independent practice association -- is the newest and fastest-growing form of HMO.

An IPA signs contracts with a large number of doctors who care for you for whatever you pay the IPA monthly. That is, they do, more or less. A patient may be faced with some co-payments or extra payments, depending on the plan.

Fifty-two readers wrote us about their recent experiences as members of M.D.-IPA, the Washington area's largest plan of this kind. Their answers were typical of scattered answers from other IPAs. Of the 52, 23 were wholly positive about M.D.-IPA, 6 mainly positive, 2 mixed, 5 mainly negative, 16 wholly negative.

The main compliments: a good doctor, good specialized care, affordability.

The main complaints: a bad doctor, a primary doctor's refusal to refer the patient to a specialist, unexpected extra charges. And in several cases, getting settled with a doctor who then became dissatisfied and dropped out of the plan.

Someone who joins a traditional HMO can often lean on "the system" for help when an individual doctor falls down. In an IPA, the patient is obviously even more dependent on getting settled with a good doctor.

"I have found the M.D.-IPA primary care physician will often honor your wishes to be referred to a particular specialist," said Richard Kopanda of Rockville. "I have been referred to about five specialists, and each has been good."

"I was able to retain the physician who had served the family for 12 years," said Portia Donovan, Gaithersburg. "That primary physician has referred me to excellent associates where necessary."

But from Harriet Reiter, Rockville: "Many doctors do not like to refer to specialists. It's money out of their pockets." And Johanna K. Stein, Washington: "{There was} delay of out-of-plan consultations by months of dickering."

And from Sandra Gibson, New Carrollton: "I have had very good care. However, I have had to find new doctors twice because the doctors I had dropped out of the plan." And Judy Ingram, Silver Spring: "I have had a very hard time trying to find a doctor who will stay with them."

IPAs, and some other HMOs, indeed have arrangements by which doctors profit or lose depending on how much expensive care their patients require. Critics say this can deprive patients of needed care. Many observers of America's rising health costs say costs can be cut only by putting individual physicians "at risk" -- financial risk -- for unneeded, over-expensive care.

Some doctors -- because they feel their bank account or their patients are suffering, or both -- do drop out of IPAs.

And many do not, patients also report.