As part of the NHLBI Twin Study, pulmonary function tests were successfully administered to 127 monozygotic and 141 dizygotic white male twin pairs 42 to 56 yr of age. Values for forced vital capacity (FVC) and forced expiratory volume in one second (FEV1) were obtained using a standardized protocol for spirometry. Initial twin analyses showed significant genetic variance (p less than 0.001) for both FVC and FEV1, whether or not adjustments were made for individual differences in age and body size. After adjustment, heritability estimates were 0.91 and 0.77 for FVC and FEV1, respectively. Further analyses indicated that the observed heritability of FVC resulted from the effects of pack-years of smoking as well as from genetic factors related to body size. These findings suggest that there were no other significant genetic determinants of FVC. In contrast, heritability of FEV1 could not be explained by constitutional factors, such as height and weight, or by cigarette smoking or propensity for cardiopulmonary disease symptoms. Additional analyses were done based on frequency of twin contact, which served as an indirect measure of environmental similarity between cotwins. Results suggested that there was a shared environmental variation in FEV1, as well as genetic variation, that could not be attributed to subpopulation differences in measured characteristics. The findings of this study are consistent with theories of genetic influences on alveolar and airway development and argue in favor of early as well as adult environmental influences on pulmonary function.