We examined shared and distinct genetic influences among standard measures of pulmonary functions: ratio of forced expiratory volume at 1 s to forced vital capacity (FEV1/FVC) and percent predicted values for forced expiratory volume at 1 s (FEV1%p), forced expiratory flow (FEFmax%p), and maximal voluntary ventilation (MVV%p) in 978-1,048 middle-aged (mean age = 55 years) male-male twins from the Vietnam Era Twin Study of Aging. A common latent factor (h(2) = 0.30) accounted for the correlations among these measures. This factor accounted for 54-81 % of the heritability of FEV1%p, FEFmax%p and MVV%p, but only explained 16 % of the heritability of FEV1/FVC. The remaining heritability of FEV1/FVC was explained by genetic influences independent of the common factor. Our findings suggest that while a common latent phenotype accounts for the relationships among different pulmonary function measures, the majority of genetic influences underlying FEV1/FVC--an index of pulmonary obstruction--are distinct from those underlying other pulmonary function measures.