As part of a larger school-based health program for Swiss apprentices and students and AIDS and sexually transmitted disease (STD) prevention exhibition was offered to 7000 boys and girls ages 15-19. Its objectives were: to reinforce their knowledge of STDs and AIDS transmission and prevention; and to modify their attitudes and behavior regarding decisions about whether when or how to have sex and the use of condoms. The exhibition consisted of 6 posters reviewed by specially trained teachers and nurses. Condoms were presented in an amusing way and were freely available. A case-control follow-up survey was conducted 12 weeks later in 2 random samples--798 subjects from an experimental school and 600 from a control school completed an anonymous multiple-choice questionnaire. 69% of the respondents were sexually active and among this group 76% of the boys and 59% of the girls had used a condom at least once but only 15% used them regularly. With regard to knowledge both groups attained high levels (75-97% correct responses); the group exposed to the exhibition scored higher for only 2 items (p < 0.05). Significantly more teens in the experimental group said they had handled condoms or had condoms with them just in case... This evaluation shows that an AIDS and STD prevention exhibition for teens may be both feasible and effective. However future intervention programs should include practical interpersonal counseling on such topics as mutual trust contraception issues and the purchase and use of condoms. (authors modified)