Using data from the Flemish Educational Assessment survey, this paper examines how AIDS knowledge and sexual activity co-vary among Flemish secondary school students, and how they are affected by education type. Results indicate that type of education affects both an adolescent´s onset of sexuality and his/her AIDS knowledge, and these effects prove robust for differences in socio-economic backgrounds. Students in lower status education types are more likely to be sexually active and to have poorer AIDS knowledge. The relationship between AIDS knowledge and sexual activity is, however, more complex. Although students in education types with poorer AIDS knowledge are more sexually active, within each of these groups the sexually active have better AIDS knowledge than the non-sexually active ones. There is also evidence of active information seeking by sexually active students which leads to improved AIDS knowledge.