Early sexual maturation has been associated with higher involvement in risk behaviour during early and mid-adolescence. In a prospective study of 2,129 girls and boys, we investigated whether the timing of sexual maturation was associated with cigarette smoking and alcohol drinking in late adolescence and whether this relation differed between boys and girls. Nine hundred and eighty boys and 1,149 girls, who participated in a cross-sectional study in middle school, were included in a follow-up study in high school 4 years later. Self-rating of pubertal status, as registered at baseline in middle school, was used to indicate the timing of sexual maturation. Age-adjusted odds ratios (ORs) with 95% confidence intervals (CIs), estimated by logistic regression, were used to assess the association between sexual maturation and alcohol drinking and daily smoking at follow-up. We found that girls who had matured early (OR 1.7, CI 1.2-2.4) or late (OR, 1.5, CI, 1.1-2.2) were both more likely to report more advanced drinking in late adolescence than were girls who were on time. Boys who had matured late were less likely (OR 0.5, CI 0.3-0.8) than boys who were on time to engage in advanced drinking. In general, daily smoking was more common among girls than boys, and more common among girls who had matured early (OR 1.5, CI 1.1-2.2) than among girls who were on time. Adjustment for social factors, e.g. parental education and marital status and parental drinking and smoking habits, did not substantially influence these results. We concluded that, for girls, but not for boys, early sexual maturation was associated with more advanced drinking and higher frequency of smoking in late adolescence. In boys, late sexual maturation was associated with reduced risk of advanced drinking.