Familial aggregation of forced expiration (as measured by forced expiratory volume in 1 sec (FEV1) and the ratio of this to total forced vital capacity (FEV1/FVC) was analyzed in 439 adult members of 108 families ascertained through control patients who had participated in a genetic and epidemiologic study of chronic obstructive pulmonary disease. Residual values for both FEV1 and FEV1/FVC obtained from regression on age, sex, race, and cigarette smoking (and height for FEV1) were used in a variance components analysis to assess the relative importance of genetic and nongenetic factors influencing familial aggregation of pulmonary function among adults. For both residual FEV1 and residual FEV1/FVC, the "best" model among a series of genetic and nongenetic models was a simple additive genetic model. A modified score test, which is robust to the assumption of multivariate normality, was used to test the significance of these estimated components. Under the most parsimonious model, additive genetic variation accounted for 28% of the variation in residual FEV1 in 108 families and 24% of the variation in residual FEV1/FVC. After outlying individuals were identified by examining goodness-of-fit statistics, the simple genetic model still gave the best fit to these data. There was little indication of non-normality in FEV1 in these families; however, FEV1/FVC did show evidence of non-normality when examining goodness-of-fit statistics. This genetic component contributing to the distribution of forced expiration may be a factor in the familial aggregation of certain respiratory diseases.