As part of the Minnesota Study of Twins Raised Apart, we measured pulmonary function in 15 pairs of monozygotic twins and one set of monozygotic triplets who were separated at an average age of 3.8 months. Six pairs of twins were concordant for nonsmoking, 3 pairs and the triplets were concordant for cigarette smoking, and 6 pairs were discordant for respiratory problems (5 pairs for smoking, 1 pair for asthma). Thirteen pulmonary function tests were done on each twin; however, only 2 tests of pulmonary function, the FEF25-75% and V75, could separate smokers from nonsmokers to a statistically significant degree. One additional test, the FEV1, was the most sensitive way to separate smoking from nonsmoking twins when only paired data from twin pairs discordant for smoking were used. Intratwin differences between twins of a pair for these three pulmonary function tests were small and virtually identical for both concordant groups, whereas the intratwin differences in the discordant group were large and distinctly different from the concordant groups. These results, together with those of previous studies in identical twins raised together, support the conclusion that genetic factors are important in determining susceptibility to airway obstruction from cigarette smoke.