Gene–environment interactions and inheritance of asthma were studied in 16-yr-old twins and their parents who participated in the nationwide Finnish Twin Cohort Study. Between 1991 and 1994, questionnaires, including a question on physician-diagnosed asthma, were mailed to the members of 2,483 twin families. The individual response rate ranged from 82 to 93%. Information on parental asthma status allowed the genetic modeling of asthma data in two different groups of twins. In families where one of the parents was asthmatic, as much as 87% of the variation in susceptibility to asthma in twins was explained by genetic factors. On the other hand, for twins whose parents were unaffected, a model including environmental effect alone was sufficient to explain the development of asthma. Genetic influences could not be totally excluded, but their role was significantly smaller. These results indicate that the presence of asthma in successive generations is more likely caused by shared genes than shared environmental risk factors; however, substantial heterogeneity among families may exist. Genetic analysis, especially among the families with an obvious familial component in development of asthma, may enhance the chances of revealing the pathogenetic mechanisms involved.