Family resemblance for normal pulmonary function as measured by forced vital capacity and one second forced expiration volume is assessed using a path analysis model which incorporates sex differences in transmissibility of the phenotype from parents to offspring as well as in the effect of a correlated shared-sibling environment. Application of this model, called XTAU, to familial correlations indicates that transmissible factors, which may be genetic or cultural, accounts for 20-30% of the variation in these measures. Further, a pronounced same-sex-specific effect of the sibling environment is indicated which enhanced the observed correlation between same-sex siblings and diminished the observed correlation between opposite-sex siblings. These results are consistent with findings of twin studies of pulmonary function indicating high heritability for both FVC and FEV1.0. In addition, the complex multifactorial model of family resemblance for normal pulmonary function is shown to have implications for specifying causal models of pulmonary disorders such as asthma, bronchitis, allergic rhinitis, and chronic obstructive pulmonary disease.