Population studies have demonstrated that obstructive airways disease aggregates within families. The authors used a twin family model of analysis to estimate the genetic and environmental influences on pulmonary function. A total of 1,635 members of 414 families of adult twins (252 monozygotic, 162 dizygotic) enrolled in the Greater Boston Twin Registry were studied between 1981 and 1982. Correlations in levels of forced expiratory volume in one second (FEV1) and forced vital capacity (FVC), adjusted for age, sex, height, and current smoking status, were compared among 16 groups of relatives sharing various degrees of genetic relatedness. A direct relation between shared genotype and the magnitude of the familial correlations for pulmonary function was observed. For FEV1, the correlations were 0.71 for monozygotic twins (100% shared genotype), 0.16 to 0.29 for relatives with 50% shared genotype, 0.09 to 0.27 for relatives with 25% shared genotype, 0.06 for cousins with 12.5% shared genotype, and -0.14 to 0.14 for unrelated family members. Correlations for FVC were similar. Stratification of the analysis by concordance or discordance for passive tobacco smoke exposure or for frequency with which families visited one another did not systematically alter these relations. These data suggest that phenotypic similarities in pulmonary function relate directly to genetic similarities, and are consistent with a multifactorial mode of inheritance.