''This is the first time we've shown that the more kids are exposed to sex in media the earlier they have sex,'' said Jane Brown of the University of North Carolina, chief author of the report.

Previous research had been limited to television, said the study which looked at 1,017 adolescents when they were aged 12 to 14 and again two years later. They were checked on their exposure during the two years to 264 items -- movies, TV shows, music and magazines -- which were analyzed for their sexual content.

In general, it found that the highest exposure levels led to more sexual activity, with white teens in the group 2.2 times more likely to have had intercourse at ages 14 to 16 than similar youngsters who had the least exposure.

The effect was not as pronounced for blacks, the study said, perhaps because the black youngsters in the study were already more sexually experienced than the whites were when the research began and thus were less influenced by media exposure over the two-year period.

The teenage pregnancy rate in the United States is three to 10 times higher than that found in other industrialized nations, making that and exposure to sexually transmitted infections a major public health concern, the study said.

At the same time, parents tend not to talk about sex with their children in a timely and comprehensive way, leaving a vacuum in which the media may become a powerful sex educator, providing ''frequent and compelling portraits of sex as fun and risk free.''

''Interestingly one of the strongest predictors of risk for early sexual intercourse for both black and white teens (in the study) was the perception that his or her peers were having sex,'' the report said.

Youngsters ''may begin to believe the world view portrayed and may begin to adopt the media's social norms as their own.

Some, especially those who have fewer alternative sources of sexual norms, such as parents or friends, may use the media as a kind of sexual superpeer that encourages them to be sexually active,'' the report added.

The study was published in the April issue of ''Pediatrics,'' the journal of the American Academy of Pediatrics. A portion of the data was previously published in the Journal of Adolescent Health.

The study was done in several schools in North Carolina.

The authors said that they did not measure the impact of exposure to sexual material on the Internet because when the research began in 2001 relatively few of the early adolescents in the sample had Internet access.