The behaviour of adolescents is recognised increasingly as having substantial and long-term consequences for their health. We examined the meaning of 'adolescence' in southern Uganda with HIV-positive young people aged 11-24 years. Adolescent girls and boys are described differently in the local language (Luganda). Adolescence is described as a behavioural rather than a life course category and an inherently dangerous one. The practices, risks and consequences of 'adolescent' behaviour are highly gendered. Local understandings of adolescence are likely to have a significant impact on the efficacy of interventions designed to minimise their 'risky behaviour'.