BACKGROUND Whether the effect of gender on the risk of first intercourse in influenced by adolescents ethnicity has received limited attention in research on age at first sex. Such information could provide a more complete understanding of adolescent sexual behavior. METHODS Life-table analysis using data from a population-based, ethnically diverse sample of 87/Los Angeles County youths was employed to estimate the median age at first sex for each gender-and-ethnicity group. Multivariate analysis using proportional hazards techniques was conducted to determine the relative risk of sexual activity among teenagers in each group. RESULTS Overall, the teenagers in the sample had a median age at first sex of 16.9 years. Black males had the lowest observed median (15.0), and Asian American males the highest (18.1); white and Hispanic males, and white and black females, reported similar ages (about 16.5 years). Hispanic and Asian American females had rates of first sex about half that of white females, although these protective effects were explained by differences in family structure. Even after controlling for background characteristics, black males had rates of first sex that were about 3-5 times the rates of the other gender-and-ethnicity groups. In addition, Asian American males were less likely than Hispanic males to be sexually experienced, and Hispanic males had almost twice the rates of sexual activity of Hispanic females. CONCLUSIONS Socioeconomic conditions account for ethnic differences among females in the age at first sex, and cultural influences may contribute to the difference between Hispanic males and females; explanations for black males, however, remain elusive.