The high level of HIV infection in sub-Saharan Africa has led to an increased interest in understanding the determinants of sexual activity among young people, who are at high risk of sexually transmitted infections. The present study examined sociodemographic, behavioral, and psychosocial factors associated with heterosexual activity among a sample of 3556 male and female high-school students in Nairobi, Kenya. Approximately 50% of the males and 11% of females reported having had sexual intercourse at least once in their lifetime with a significant proportion reporting multiple sexual partnerships. Sexual activity was associated with various factors including religiosity, perceived parental attitudes towards sex, living arrangements, and school characteristics. However, the pattern of association differed for males and females. Results suggest that adolescents may benefit from sex education programs addressing multiple factors that may predispose adolescents to sexual activity, and that take into account gender differences. (Contains 5 tables.)