Both the prevalence of adolescent smoking and the future health consequences of the behaviour are well known. Studies bearing on the psychological reasons why adolescents smoke have, however, relied largely on cross-sectional collections of data, and knowledge in the area has accordingly been restricted, by and large, to reports of psychological correlates of the behaviour. This study investigated psychological reasons associated with the onset of adolescent smoking behaviour in a large cohort of initial nonsmokers (1,945 males and 1,716 females) aged 13 to 17 years, examined in detail at intake and then followed up over one year. Data were collected by self-completed questionnaire given under supervision in school time. Predictors of adolescent smoking onset were similar for boys and girls, and for all age groups, and centred on both expectations of future smoking and the social climate within which smoking onset takes place. Briefly, adolescents are more likely to smoke if they expect to do so, function within a social climate which allows smoking, and report exemplar pressure to smoke. These prospective associates of smoking onset in adolescents point to the possibility of more closely focused programs for the prevention of adolescent smoking behaviour.