Data from the American Medical Association show that in each of 43 states, a handful of top insurers have gained such a stronghold that their markets are considered "highly concentrated" under U.S. Department of Justice guidelines, often far exceeding the thresholds that trigger antitrust concerns.

The study also shows that in 166 of 294 metropolitan areas, or 56 percent, a single insurer controls more than half the business in health maintenance organization and preferred provider networks underwriting.

"This problem is widespread across the country, and it needs to be looked at," said Jim Rohack, an AMA trustee and physician in Temple, Texas. "The choices that patients have now are more difficult."

The AMA study cited a Justice Department benchmark in citing antitrust concerns, the Herfindahl-Hirschman Index, or HHI. A score above 1,000 shows "moderate" concentration. Those scoring above 1,800 yield a "high" concentration.

Figures show that 95 percent of the 294 HMO/PPO metropolitan markets studied were above 1,800. Raise that HHI bar even higher to 3,000, and 67 percent rise above it.

The AMA study is the latest piece of evidence — and most comprehensive to date — showing the market power of a few companies, and a large number of regional nonprofit Blue Cross operations, is formidable and growing. And it comes as premiums continue to grow at near double-digit percentage rates.

Critics say that carriers are not only creating monopolies and oligopolies in many regions, they also control the other side of the equation in what is known as monopsony power. That means in addition to having the most enrollees, they're also the biggest purchasers of health care and can dictate prices and coverage terms.

It also makes it harder for new carriers to emerge, as pricing already has been set by the dominant carrier.

That's particularly true in North Dakota, where the state's Blue Cross Blue Shield provider has, by various estimates, a roughly 90 percent share of the market, said Insurance Commissioner Jim Poolman. New carriers would have to pay more to health-care providers and charge less to policyholders to gain a foothold.

In North Dakota, there isn't much incentive for that, he added.

"It's difficult in a market of 640,000 people to write new insurance policies," Poolman said.

The AMA says there have been more than 400 mergers among health-care insurers in the past decade. As they've consolidated and presumably eliminated duplicative functions, they're not passing the savings in personnel and administrative costs on to consumers. Rate increases, though slowing, are higher than ever and growing at a near double-digit pace.

Studies by the Kaiser Family Foundation show double-digit premium hikes from 2001 to 2004 — peaking with a 13.9 percent jump in 2003 — soared well above inflation and wages. Those categories have risen at rates less than a half to less than one-fifth that of insurance premiums, Kaiser says.

Last year, the string of double-digit jumps was broken, but was close to that level with a 9.2 percent increase, the Kaiser study said. The foundation is not affiliated with the nonprofit HMO of the same name.

Some health insurance analysts have said the recent uptick in premiums is part of an "underwriting cycle," in which carriers go through a period of boosting profits, and then ease up on premium increases for several years.

But Gary Claxton, vice president at the Kaiser Family Foundation, contends fewer insurers mean the need for underwriting cycles has diminished, and it's likely that carriers will settle on the high side when it comes to premium increases.

"They won't get down to cost," he said. "They see it as their collective right not to cut prices too much."

David Colby, chief financial officer for WellPoint Inc., the nation's largest carrier, disagreed. He said medical cost increases have forced his company to hike premiums. He added that the percentage his company spends on actual medical care has remained constant in recent years.

"Our premiums are pretty much tracking what medical costs are doing," he said.

The AMA says it has taken up this antitrust issue with the Department of Justice, but says it has run into roadblocks with regulators. AMA officials say regulators seem uninterested, even though government officials are more than willing to target doctors' groups and hospitals on antitrust matters.

Justice Department officials did not respond to requests for comment.