The study was designed to evaluate the impact of education on AIDS knowledge among prison inmates in Maputo, Mozambique. A 6-month follow-up study was carried out in 1993 among 300 prisoners. A knowledge, attitudes, and practices questionnaire regarding AIDS and STD was administered to each subject as part of the intake medical examination and after an educational intervention provided by 30 prisoner 'activists'. A large proportion of prisoners had high risk behaviours (65% had 2 or more sexual partners per month and 39% had a history of STD) and low AIDS knowledge at incarceration. Statistically significant increases in knowledge occurred after the intervention. Prisoners with less formal education had a poorer performance on the initial questionnaire (43% vs 69% P < 0.00001) and had a greater improvement after the intervention (41% vs 24%, P < 0.00001). The results demonstrate that educational interventions involving peer health educators contribute positively to the acquisition of knowledge among prisoners.