WellPoint, the nation's largest publicly traded health insurer with 34 million customers, said the dependent provision was responsible for adding 280,000 new members. That was about one third its total enrollment growth in the first three months of 2011. Others large insurers said they have added tens of thousands of young adults. Aetna, for example, added fewer than 100,000; Kaiser Permanente, about 90,000; Highmark Inc., about 72,000, and Health Care Service Corp., about 82,000.

The Health and Human Services Department has estimated that about 1.2 million young adults would sign up for coverage in 2011. The early numbers from insurers show it could be much higher, said Aaron Smith, executive director of the Young Invincibles, a Washington-based nonprofit group that advocates for young adults.

Insurers described the growth in young-adult enrollment as the industry began reporting first-quarter earnings that showed better than expected profits. But Carl McDonald, an analyst for Citigroup, said that the higher profits weren't related to the new young-adult enrollees. That's because, he said, most of the increase in young people's enrollment has occurred among self-insured employers; in those firms, insurers act as administrators and don't not assume financial risk.