A major source of health insurance for people who work for themselves has all but disappeared, casting thousands of contractors, freelancers and solo practitioners into the ranks of the uninsured with little hope of obtaining new coverage.

Health plans offered by professional associations were once safe havens for millions of people who couldn't obtain coverage anywhere else. But, as medical costs have soared, groups representing professions as varied as law and golf have been forced to stop offering the benefit or have been dropped by insurers.

While no one tracks association coverage to know how many plans have disappeared, the experience of Marsh Affinity Group Services is telling. A decade ago, Marsh, which brokers and administers such plans, had 142 such clients. Today, all but three have closed.

During the same period, the uninsured U.S. population, now estimated at 45 million, rose dramatically, fueled in part by the dearth of affordable options for the self-employed, experts said. Among uninsured workers, nearly 63 percent are self-employed or work in small companies, Todd Stottlemyer, president of the National Federation of Independent Business, told Congress recently.

Fewer than a quarter of the 1,020 professional and small-business associations surveyed in February offer medical coverage, even though a majority of the groups said they would like to. The American Society of Association Executives, which commissioned the survey, views the issue as a crisis.

In its heyday, association health coverage was so popular it was touted as a membership recruiting tool for professional organizations.

"The association business used to be a huge part of the group health insurance business," said Robert Laszewski, a Washington-based health-policy consultant and former insurance executive. "Now it's like the buggy and whip business--almost entirely gone."

Insurance carriers began pulling out of association markets about 10 years ago amid mandates requiring the groups--like employers--to offer coverage to all members who wanted to buy it, regardless of pre-existing conditions. Unlike employers, however, who typically pick up the lion's share of premiums for employees, most associations do not share in the costs. Instead, they arrange for their members to buy coverage at group, rather than individual, rates.

In today's marketplace, that's almost always a better deal for older members and often the only option for people with pre-existing conditions. But insurers are eager to sell individual policies to the young and healthy for as little as $100 a month, scooping the cream off the risk pool. That leaves higher-risk older and sicker people to the group market, a phenomenon known as adverse selection.

As healthy members leave an association health plan, the concentration of members with higher-than-average medical costs increases. That forces the underwriter--usually the insurer but sometimes the association--to raise premiums. A "death spiral" sets in, when medical costs exceed the plan's ability to raise premiums to cover them.

"The problem with associations is they go into a death spiral because they get the worst risk," said Alan Fox, vice president of plan design for the American Psychological Association Insurance Trust, which covered thousands of psychologists and their families for 35 years before closing its health plan in 1999.

The list of casualties also includes health plans once sponsored by the American Bar Association, which still hopes to resurrect the benefit it dropped last year, and the California Bar Association, which lost its coverage when its insurer pulled out in the early 1990s.

Before the Professional Golfers' Association's health plan ran into the rough, the group had extended coverage to about 1,000 golfers. But the plan was discontinued in 1996, as medical costs rose and younger, healthier members bought coverage on their own at lower rates.

"If you can get cheaper coverage through the individual market, that's what you do," said Mila Kofman, an associate research professor at Georgetown University's Health Policy Institute.

But not everybody can buy an individual plan. In many states, insurers are allowed to reject applicants for individual policies for any medical reason, including common conditions such as asthma and varicose veins. As a result, many people who lose association coverage are effectively uninsurable.

Insurance options of last resort--Cobra-conversion coverage, named after the federal law that created it, and publicly subsidized high-risk pools--are not an option for everybody because the coverage is insufficient or unaffordable, or both.