Nearly half the African-Americans in the study of teenagers ages 14 to 19 were infected with at least one of the diseases monitored in the study - human papillomavirus (HPV), chlamydia, genital herpes and trichomoniasis, a common parasite.

The 50 percent figure compared with 20 percent of white teenagers, health officials and researchers said at a news conference at a scientific meeting in Chicago.

The two most common sexually transmitted diseases, or S.T.D.'s, among all the participants tested were HPV, at 18 percent, and chlamydia, at 4 percent, according to the analysis, part of the National Health and Nutrition Examination Survey.

Each disease can be serious in its own way. HPV, for example, can cause cancer and genital warts.

Among the infected women, 15 percent had more than one of the diseases.