Retrospective information about teenage alcohol use was obtained from 1589 adult twin pairs aged 20-30 years from the Australian twin register. Twin pairs were highly concordant both for teenage drinking (or abstinence) and for early versus late onset of drinking. Sociodemographic variables (e.g., paternal occupation and parental religious affiliation) and psychosocial variables (e.g., personality and attitudinal traits), assessed when the twins were adults, were comparatively poor predictors of teenage drinking. Environmental influences on onset of drinking appeared to be sex-specific, i.e., uncorrelated over twins from unlike-sex pairs. Among drinkers, early versus late onset of drinking was more strongly influenced by inherited factors in females, but by shared features of the social environment (e.g., family background or school experience) in males.