RESUMEN
OBJECTIVE: to analyse adolescents' knowledge of preventive sexual practices related to human immunodeficiency virus (HIV) and acquired immunodeficiency syndrome (AIDS) by means of a questionnaire recommended by the Joint United Nations Programme on HIV/AIDS (UNAIDS). METHOD: non-experimental, cross-sectional, descriptive and analytical investigation. SETTING: four schools in Chillán, Chile, 2005. PARTICIPANTS: a total of 480 adolescents aged between 15 and 19 years. Students completed a questionnaire recommended by UNAIDS in order to develop basic indicators. FINDINGS: the indicator of preventive sexual practices related to HIV/AIDS was 32.5%; forms of prevention (62.5%) were better known than erroneous ideas about transmission (46%). Adolescents from the only private school in the study demonstrated greater knowledge (43.3%) than students from the public schools (25%) (p<0.01). No significant differences were found in knowledge according to age, sex or educational level. CONCLUSION: adolescents have poor knowledge of preventive sexual practices related to HIV and AIDS. It is necessary to implement an indicator of knowledge that allows for the creation and monitoring of sexual education programmes.