A multivariate twin study was conducted in order to evaluate to what extent smoking, BMI and longevity are influenced by common genetic factors. The study was based on a 28-year follow-up of a sample of 2464 Danish twins who were born in the period 1890-1920 and who answered a questionnaire, including requests for information on smoking status, height and weight, in 1966. By 1994, approximately 2/3 of the sample had died. To compensate for the right-censoring, age at death was imputed for twins who were still alive by using survival analysis; all living subjects were more than 73 years old (mean 80 years, SD 5) in 1994. Proportions of covariance resulting from genetic and environmental factors in common and unique to the three traits were estimated from covariance matrices using the structural equation model approach. The study found no evidence for a substantial impact of common genetic factors on smoking, BMI and longevity. This suggests that only a small fraction of the genetic influences on longevity is mediated via a genetic influence on smoking and BMI and, furthermore, that it is unlikely that the associations between smoking and mortality and between BMI and mortality are confounded by common genetic factors.