"These new HMOs that dominate the market are primarily for-profit," Woolhandler said. "And little is known about the quality of care in those plans."

Woolhandler said the results confirmed what many doctors had believed all along.

"I think a lot of doctors had guessed that the quality of care is better in non-profit HMOs," she said, adding that private health plans generally spend more money than not-for-profits on various expenses.

The study examined 248 private plans and 81 not-for-profits, finding that not-for-profits had an overall higher quality of care in areas such as immunization and prenatal care.

The researchers worked with data the HMOs turned over to the National Committee for Quality Assurance. And Woolhandler said the data, which was volunteered by the HMOs themselves, is as accurate as possible.

"We think they may try to make themselves look good," she said. But she added that "there are rules on how they collect the data and some of it is audited."

Woolhandler said that while she and her co-author are interested in pursuing the subject further, such studies have been made more difficult recently by HMOs' refusal to hand over data about their care.

"The quality of the info available in the future is likely to be less than it is now," she said. "It's almost impossible to get information on HMOs that refuse to release their data."