Aims: To identify with whom in-school adolescents preferred to communicate about sexuality, and to study adolescents' communication on HIV/AIDS, abstinence and condoms with parents/guardians, other adult family members, and teachers. Data were obtained from a baseline questionnaire survey carried out in South Africa (Cape Town and Mankweng) and Tanzania (Dar es Salaam) in early 2004. We analysed data for 14,944 adolescents from 80 randomly selected schools. The mean ageswere as follows: CapeTown, 13.38 years (standard deviation (SD) 1.15); Mankweng, 13.94 years (SD 1.35); and Dar es Salaam, 12.94 years (SD 1.31). Results: Adolescent females preferred to receive sexuality information from their mothers, while among males there was a higher preference for fathers in two sites. Thirty-seven per cent, 41% and 29% reported never or hardly ever communicating about sexuality with parents, other adult family members, and teachers, respectively. 'Silence' was more prevalent in Dar es Salaam than in the other two sites.