The effect of a 1-hr school-based AIDS/HIV education program on the knowledge and attitudes of high school students was evaluated with a modified version of the Centers for Disease Control Health Risk Survey. One urban and one suburban school each were randomly assigned to an educational intervention (n = 535) or a control group (n = 659). All students received a posttest 2 weeks after the intervention. Knowledge was based on responses to 12 true-false questions (pretest alpha = .76, posttest alpha = 0.81). Principal components analysis was used to develop three attitude scales and risk-taking behavior was assessed by self-report. Data were analyzed with Kruskall-Wallis analysis of variance (ANOVA) and multivariate ANOVA. The groups did not differ in knowledge level at pretest. At posttest the education group had significantly (p < or = 0.006) higher knowledge even after controlling for the effects of previous AIDS education (p < or = 0.019), gender (p < or = 0.007), and Hispanic ethnicity (p < or = 0.048). After the education program, students were less worried about exposure to the AIDS virus, but were more worried (p < or = 0.048) about AIDS acquisition during their adult life. Although single school-based AIDS/HIV education programs may increase knowledge, more extensive education may be needed to change the behavior and attitudes of older high school students.