The authors studied familial aggregation of mortality risk factors and their association with future deaths in the Finnish Twin Cohort. Cohort members (n = 15,904) aged 24-60 years and healthy at the end of 1981 who had responded to questionnaires in 1975 and 1981 were followed up for death through June 30, 2001. In individual-based analyses, the age- and sex-adjusted risk of death was higher among persons who were not participating in vigorous leisure physical activity in 1975 and 1981 than in those who were (p < 0.001), and risk was increased among smokers (p < 0.001) and heavy users of alcohol (p < 0.001). The study sample included data on both members of 3,551 dizygotic same-sex twin pairs and 1,772 monozygotic same-sex twin pairs. In pairwise analyses of discordant dizygotic pairs, risks of death were increased among co-twins who were not participating in vigorous activity (p = 0.05), co-twins who were smokers (p < 0.001), and co-twins who were heavy alcohol users (p < 0.001). Correspondingly, among monozygotic twin pairs, a difference in risk was seen only for smoking (p = 0.03). The authors conclude that, after adjustment for childhood environment, a low level of leisure physical activity, smoking, and heavy use of alcohol are associated with increased risks of death, but genetic selection may account for some of the association concerning physical activity and heavy use of alcohol.