Using a correlated liability dimensions model, we examined the extent to which the same genetic and environmental factors influence both initiation of regular cigarette smoking and maintenance of the smoking habit in men and women. We analyzed questionnaire survey data obtained from large samples of male and female like-sexed twins from three countries, Australia (N = 1535 pairs), Sweden (N = 5916 pairs), and Finland (N = 4438 pairs), subdivided into three age bands (18-25, 26-35, and 36-46 years of age). We found that familial influences on risk for persistence in smoking cannot be entirely explained by the same factors responsible for risk of smoking initiation. Total genetic variance for smoking persistence varied little by age band and sex (range, 39-49% in women and 42-45% in men); however, even among twins in the youngest group (18-25 years of age), who on average have the fewest years of cigarette use, less than 40% of the total genetic variance in smoking persistence was accounted for by the same genetic factors that increased risk of smoking initiation, and this percentage decreased to less than 10% in the 36-46 year olds.