Objectives: To assess the sexual- and drug-use-related risk behaviours of male juvenile detainees in Karachi, Pakistan. Design: Cross-sectional study. Methods: A cross-sectional study was conducted of prison inmates aged 15-21 years in Karachi Juvenile Prison in 2002. In total, 321 inmates were interviewed about sexual orientation and behaviours, and knowledge about human immunodeficiency virus (HIV) and sexually transmitted infections (STIs). Urine specimens were collected and tested for Chlamydia trachomatis and Neisseria gonorrhoeae using ligase chain reaction. Results: A substantial proportion (n = 111, 34.6%) of the participants were sexually active. Sixty-two (19%) and 67 (21%) had had sex with a male or female before incarceration, respectively. Twenty-seven (8.4%) participants had an STI, and 50% of the 109 sexually active participants had had multiple sexual partners. Use of addictive substances was associated with sexual activity. The mean knowledge score computed from the sum of 16 items was 4.7, with a median of 2.9. A large proportion (40%) of participants knew about condoms, but very few (3.4%) had ever used one. The mean ± standard deviation risk score from nine items was 2.4 ± 1.7. On the basis of behavioural and biological markers, 117 (36.4%) participants had high-risk behaviour. In multivariate logistic regression analysis, knowledge, risk perception and age were predictive of higher risk. Conclusions: HIV risk behaviours are common among adolescent inmates. Although inmates do have knowledge about modes of transmission and condom use, the use of condoms is significantly low. Interventions are needed for behavioural change among this group. Crown Copyright © 2009.