604 out of 700 students (330 females, 274 males; mean age 23.3 [19-64] years) completed a questionnaire, which was handed out by their teachers and consisted of 26 questions concerning sexual practices and knowledge about AIDS. 285 subjects were studying natural sciences, 134 arts and 185 medicine. There were significant differences between the sexes (P less than 0.01) in drug use (alcohol, cannabis), use of condoms, promiscuity and with respect to discussion of AIDS. Fewer females than males said that they occasionally used intoxicants before sex (14% vs 25%). Women also used condoms less frequently than men (23% vs 44%), although more insisted on their use if their partner declined (68% vs 58%). 55% of the men and 72% of the women were living within a stable relationship. More women (70%) than men (56%) said they would discuss AIDS before having sex with a new partner. There were significant differences between students from individual disciplines (P less than 0.01) with regard to their knowledge, use of condoms, promiscuity, and discussion of AIDS: arts students were less well informed and changed sexual partners more frequently. The same was true of students more than 30 years old. There are important differences between students with regard to their knowledge about AIDS and sexual behaviour risk factors. Educational programmes need to take this fact into account.