This study evaluated the effects of an AIDS/STD curriculum for 9th- and 10th-grade students in the Netherlands. Curriculum development was based on (1) theory-based need assessments among students and teachers, (2) pilot testing of data-based and theory-based methods and materials, and (3) cooperation between researchers and students, teachers, and gatekeepers within the school system. Using a quasi-experimental design, program effects on students' attitudes, beliefs, and sexual behavior were compared with those of current AIDS/STD education practice. The results indicated that the experimental curriculum had a stronger favorable impact on students' attitudes and beliefs regarding using condoms consistently. Regarding sexual risk behavior, a differential curriculum effect could be demonstrated. These findings support the contention that current AIDS/STD education can be improved by (1) using empirical data, (2) applying multiple theories from the social sciences, and (3) involving representatives within the school system in the development process.