Reducing the Risk is a new sexuality education curriculum, based on social learning theory, social inoculation theory and cognitive-behavioral theory and employing explicit norms against unprotected sexual intercourse. In a quasi-experimental evaluation, this curriculum was implemented at 13 California high schools; 758 high school students assigned to treatment and control groups were surveyed before their exposure to the curriculum, immediately afterwards, six months later, and 18 months later. Among all participants, the program significantly increased participants' knowledge and parent-child communication about abstinence and contraception. Among students who had not initiated intercourse prior to the pretest, the curriculum significantly reduced the likelihood that they would have had intercourse by 18 months later. Reducing the Risk did not significantly affect frequency of sexual intercourse or use of birth control among sexually experienced students. Among all lower risk youths and among all students who had not initiated intercourse prior to their exposure to the curriculum, the curriculum appears to have significantly reduced unprotected intercourse, either by delaying the onset of intercourse, either by delaying the onset of intercourse or by increasing the use of contraceptives. Among the students not sexually active before participation in the program, effects seem to have extended across a variety of subgroups, including both whites and Latinos and lower risk and higher risk youths, but were particularly strong among lower risk youths and females.