Studying monozygotic and dizygotic adoles-cent twin pairs of both sexes reared together, the present study examined the extent to which the variance in smoking involvement is attributable to genetic and envi-ronmental effects, and to what extent there are sex differ-ences in the etiology. Questionnaire data on how often the adolescent had ever smoked tobacco was collected from a population-based twin sample consisting of seven national birth cohorts (ages 12–18), their mothers, and their fathers (N = 1,394 families). The data was analyzed with multi-variate genetic modeling, using a multi-informant design. The etiological structure of smoking involvement was best represented in an ACE common pathway model, with smoking defined as a latent factor loading onto all three informants' reports. Estimates could be set equal across sexes. Results showed that adolescent lifetime smoking involvement was moderately heritable (37 %). The largest influence was from the shared environment (56 %), while environmental effects unique to each twin had minimal influence (7 %). Keywords: Tobacco smoking, Twin study, Adolescents, Genetic and environmental effects, Heritability, Multi-informant