Despite their increasing numbers, few of the sexuality education and pregnancy prevention programs in developing countries have been evaluated. This study, conducted in 1995-1997, assesses the impact of a school-based sexuality education program, the Grade 7 Project, on 945 Jamaican seventh graders (aged 11-14) and their initiation of sexual activity and use of contraception at first intercourse, as well as the knowledge and attitudes that influence their behaviors. Using a quasi-experimental design, the study measured the effects of the Grade 7 Project when the nine-month intervention was completed (short term) and one year after that (long term). Multivariate logistic regression analysis indicated that the project had no effect on initiation of sexual activity, but it had a positive short-term impact on use of contraception at first intercourse (P = .08); adolescents in the intervention group were more than twice as likely to use contraception. The project also had a positive short-term influence on several aspects of the adolescents' knowledge of and attitudes about sexuality and pregnancy. The modest impact of the Grade 7 Project is encouraging, as school-based sexuality education programs of limited duration rarely have a long-term impact. Moreover, competing socioeconomic and cultural forces in Jamaica encourage early sexuality and parenthood among adolescents. The use of more participatory teaching methods and smaller class sizes might strengthen the Grade 7 Project and enhance its impact.