BACKGROUND. Effective prevention of smoking depends on the identification of factors that determine smoking onset. We examined the influence of family factors during childhood (household income, parents' education and smoking behaviour) on the subsequent risk of smoking in young adults. METHODS. In 1979, 1300 children aged 6-18 years, whose parents were randomly selected for participation in the Copenhagen City Heart Study were invited to a health examination. Information about health and smoking behaviour was obtained from 73% of the children. A random sample of 579 of the children were invited to a follow-up examination 13 years later. In all, 486 (84%) participated in the follow-up. RESULTS. The influence of household income, parents' smoking behaviour and education on the child's risk of becoming a smoker in young adulthood was estimated. Household income and parents' education did not significantly affect the risk of adult smoking. Maternal smoking during childhood increased the risk in comparison with the mother being a non-smoker (adjusted odds ratio = 1.95; 95% confidence interval: 1.07-3.58). CONCLUSION. Maternal smoking during childhood increases children's risk of becoming young adult smokers, independent of age and smoking behaviour in childhood, gender and social background. In Denmark 28% of smoking in young adults could be attributed to maternal smoking.