The major results are presented of a study of adolescent sexuality in five Sahel countries: Burkina Faso, Gambia, Mali, Niger, and Senegal. Because of strong taboos on discussing sexuality, most studies of reproductive health in the region have paid little attention to adolescents, who constitute over one-fifth of reproductive-age women. Awareness of problems in adolescent reproductive health is limited. Marriage age in the five countries is among the lowest in the world. In urban areas marriage age is increasing, but premarital sex is becoming more common. 51% of uneducated rural girls in Niger are married by age 15, as are 26% who are educated. But at age 20, 38% in Ouagadougou, 52% in Niamey, and 71% in Dakar are still single. Early marriage in the Sahel is usually followed rapidly by a pregnancy in an immature adolescent. The medical consequences of early pregnancy are a public health problem: spontaneous abortion, premature or difficult deliveries, high cesarean rates, infections, fistulas, trauma to the newborn, and low birth weight. Premarital sexual activity carries the same risks of early pregnancy, with the additional social and economic consequences inherent in non-marital fertility. Unwanted pregnancy, illegal abortion, or even infanticide may occur. The proportion of single mothers under age 20 varies from 9% to 18% in the large cities of the Sahel. Adolescents appear to be especially vulnerable to sexually transmitted diseases and HIV infection in case of unprotected sex, possibly because of their physical immaturity. Knowledge of sexually transmitted diseases is limited among girls, and most do not know that seemingly healthy persons can be HIV seropositive. Friends and the media are the most common sources of information about sex, and health agents, family members, and teachers are among the least frequent sources. Most older respondents agreed that premarital sexual activity has increased. Various explanations including later marriage and economic problems were advanced. Some parents implicitly approve of contraceptive use by adolescents, but others feel that it would encourage sexual activity. According to adolescents themselves, the disapproving attitude of health workers prevents them from seeking contraception and other needed reproductive health services.