Most studies on sexual behavior have approached the relationship between AIDS knowledge and sexual behavior unidirectionally. This paper sets out to examine a reciprocal relationship between AIDS knowledge and sexual behavior, in which it is possible that adolescents who enter into sexuality may start to actively seek out information on sex. Additionally, the effects of various sources for AIDS-related information and condom use are analyzed. Data from the Flemish Educational Assessment project, which surveyed a sample of adolescents from the 3rd and 5th years of secondary school, were used. Results indicate that the relationship between AIDS knowledge and sexual behavior is quite complex. On one hand, we find that better knowledge leads to a later onset of sexuality and more consistent condom use; on the other hand, we find that adolescents who start sex earlier and use condoms inconsistently tend to be more knowledgeable about AIDS.