This study evaluated the effectiveness of Positive Prevention, a theory-based, HIV/STD prevention education curriculum for high school youth. Three hundred fifty-three students participated in a longitudinal experimental design to determine the impact of the curriculum on HIV/AIDS knowledge, self-efficacy to abstain from sex, self-efficacy of condom use, attitudes toward abstaining from sex, and sexual behaviors. Results indicate that the curriculum significantly increased positive attitudes toward abstaining from sexual intercourse, increased self-efficacy to abstain from sexual activity, and increased self-efficacy to use condoms. Among students who had not initiated sexual intercourse prior to the pretest, the curriculum significantly reduced the likelihood that they would have initiated sexual activity six months later. These findings are consistent with the literature surrounding effective school-based prevention programs that suggests that curricula emphasizing social skills while limiting bio-medical information are effective in reducing HIV/STD risk behaviors. (Contains 3 tables and 1 figure.)