Context: Early initiation of intercourse and the context within which sexual activity begins are key indicators of adolescents' potential risk for unplanned pregnancy, abortion and sexually transmitted diseases. Comparative information on the sexual behavior of male and female adolescents in different countries assists health planners and service providers in meeting adolescents' needs. Methods: Data from the most recent nationally representative surveys of reproductive behavior in 14 countries throughout the world were used to assess regional variations in young people's sexual behavior. Analyses focus on 15-19-year-olds, but also use data from 20-24-year-olds to provide a more complete picture of gender differences in behavior during adolescence. Results: In most countries, roughly one-third or more of teenage women have had intercourse; in four countries (Ghana, Mali, Jamaica and Great Britain), about three in five are sexually experienced. Between about one-half and three-quarters of adolescent males in seven countries have ever had intercourse, but the proportion is one-third or less in Ghana, Zimbabwe, the Philippines and Thailand. In most countries, sexual intercourse during the teenage years occurs predominantly outside marriage among men but largely within marriage among women. Never-married young people are considerably less likely to be currently sexually active than to be sexually experienced. For example, in Ghana, 49% of never-married adolescent women have had intercourse, but only 23% have done so within the past month. Conclusions: In most of these countries, a high proportion of adolescents are potentially at risk for a range of poor reproductive health outcomes. Program planners must find ways to help sexually active adolescents consistently use effective means of protection against both pregnancy and sexually transmitted diseases.