Purpose: In a context of increasing HIV/STD risks among adolescents in Latin America, new conceptions of sequential "sex education" are needed. However, older adolescents must be reached through shorter programs. A quasi-experimental study was conducted to evaluate the impact of a short school-based STD/AIDS prevention program on knowledge, attitudes, and intended behavior among secondary students in Lima. Methods: Aimed at empowering adolescents regarding their sexuality, improving knowledge and attitudes, and developing skills and prevention-oriented behavioral intentions, the program consisted of seven weekly two-hour sessions. Cost exclusive of research expenses was $3 per student reached. Results: 1213 students from 14 schools participated in either the intervention or control groups. Significant changes in knowledge on sexuality and AIDS, erotophilia, acceptance of contraception, machismo, and discrimination against persons with HIV/AIDS were found in the intervention group, as compared to the control group. Self-efficacy and prevention-oriented behavioral intentions were significantly better in the intervention group. Conclusions: Owing to its effectiveness, acceptability, and low cost, this program may become useful in reaching adolescents unable to participate in longer-term sex education programs in Peru and elsewhere. © 1994.