Despite the rapid increases of HIV and sexually transmitted infections (STIs) in East Asia, the sex-related knowledge among Chinese adolescents is limited and the standard of sex education is both variable and inconsistent in Hong Kong. The aim of this study is to review the overall effectiveness of the school-based or doctor-based sexual health education currently provided for Hong Kong adolescents. In 1999 - 2000, a large survey on youth risk behaviours was carried out among 15 - 18-year-old students from 21 secondary schools. Of the 8,039 questionnaires, 377 students (4.7%) admitted sexual experience. Multiple logistic regression showed that students who had received school AIDS education were found to be less likely to have sexual intercourse in the past 3 months (odds ratio, OR 0.5; 95% confidence interval, CI 0.3 - 0.9) or use drugs or alcohol before the previous intercourse (OR 0.5; 95% CI 0.3 - 0.9). They were twice as likely to discuss emotional (95% CI 1.4 - 4.0) or puberty issues (95% CI 1.1 - 3.0). Although not statistically significant, sex education by family doctors showed a similar pattern. This result provides the strongest available evidence that school-based sex education programmes can modify sexual behaviour, which, in turn, reduces risk among Chinese students.