We assessed knowledge of AIDS among pupils in selected schools in Tanzania in August 1989. Four hundred and eighty-one pupils from four randomly selected secondary schools, two from Dar-Es-Salaam (a city) and two from Bagamoyo (a semi-rural town), were interviewed using a structured questionnaire. Of these, 476 (99.0%) had heard of AIDS, and 447 (92.9%) were able to mention spontaneously at least one sexually transmitted disease (STD), of whom 374 (83.7%) mentioned AIDS. Knowledge was found to increase with age and tended to be higher among women in Dar-Es-Salaam than in Bagamoyo. These data suggest that communication channels directed at women in rural areas should be strengthened. While knowledge of sexual transmission of HIV was generally high, and prevalence of reported misconceptions about modes of transmission was very low, knowledge of non-sexual means of transmission (transfusions, injections, vertical) was lacking. Although 80% of pupils mentioned reduction of number of sexual partners as a means of AIDS prevention, only 22% mentioned condom use, and less than 5% reported that they had ever used a condom. Future research should concentrate on means of promoting sexual behavior change, the ultimate aim of any AIDS prevention strategy.