The aim of the present study was to simultaneously examine the effect of factors related to school, leisure, family and the individual on adolescent smoking. These determining factors occupy the micro- and mesosystems of Bronfenbrenner's Ecological Theory. Data were collected using questionnaires from a nationally representative sample of 15-year-olds (1404 pupils) and 73 staff members from 73 schools in Norway. Males formed 51% of the sample size. Consistent with previous studies, the various factors were linked to adolescent smoking when examined separately. However, in an ecological setting, factors related to the individual and leisure (i.e., attitudes towards smoking, perceived behavioural control and evenings with friends) emerged as the strongest predictors. School and family factors were completely mediated by individual and leisure factors, although when examined alone, they explained quite a substantial amount of the variance and a considerable amount of school difference in adolescent smoking. The implications are discussed.