Objective. —To evaluate the effectiveness of a teacher-delivered curriculum in favorably modifying acquired immunodeficiency syndrome (AIDS)—related knowledge and beliefs, self-efficacy related to AIDS-preventive actions, and involvement in AIDS risk behaviors among an eligible population of 1316 New York City high school students. Design. —Students in two pairs of demographically similar high schools were assigned to receive either a special six-lesson AIDS-preventive curriculum (intervention group) or no formal AIDS-preventive curriculum (comparison group). Participants. —Study participants were ninth- and 11th-grade students, 12 to 20 years of age (mean, 15.7 years), 41.5% male, and 72.1% black or Hispanic. Intervention. —The special curriculum focused on conveying facts about AIDS, fostering theoretically derived beliefs favorable to AIDS prevention, and teaching skills necessary for the successful performance of AIDS-preventive behaviors. Main Outcome Measures. —AIDS-related knowledge, beliefs, self-efficacy, and behaviors were assessed among students in intervention and comparison groups at study baseline and at 3 months' follow-up. Results. —Significant (albeit modest) effects favoring intervention were observed for knowledge, beliefs, self-efficacy, and risk behavior scores. Conclusions. —School-based AIDS-preventive curricula may play a role in curtailing transmission of the human immunodeficiency virus among multiethnic groups of urban adolescents; however, such curricula may need supplementation by a broader-based prevention effort to achieve substantial risk behavior change. (JAMA. 1993;270:725-730.