They borrow leftover prescription drugs from friends, attempt to self-diagnose ailments online, stretch their diabetes and asthma medicines for as long as possible and set their own broken bones. When emergencies strike, they rarely can afford the bills that follow.

"My first reaction was to start laughing — I just kept saying, 'No way, no way,' " Alanna Boyd, a 28-year-old receptionist, recalled of the $17,398 — including $13 for the use of a television — that she was charged after spending 46 hours in October at Beth Israel Medical Center in New York with diverticulitis, a digestive illness. "I could have gone to a major university for a year. Instead, I went to the hospital for two days."

In the parlance of the health care industry, Boyd, whose case remains unresolved, is among the "young invincibles" — people in their 20s who shun insurance either because their age makes them feel invulnerable or because expensive policies are out of reach. Young adults are the nation's largest group of uninsured — there were 13.2 million of them nationally in 2007, or 29 percent, according to the latest figures from the Commonwealth Fund, a nonprofit research group in New York.