Pulmonary function studies were carried out in a group of asymptomatic nonsmoking adults with intermediate alpha-1-antitrypsin deficiency who were attending an early disease detection unit in Rochester, N. Y. All subjects were identified by specific protease inhibitor (Pi) typing. Fifteen MZ and 14 MS subjects who had never smoked cigarettes were matched by sex and age to MM controls. Spirometry, static lung volumes, and single breath-diffusing capacity were identical in all Pi type groups with no statistically significant differences noted. Maximal expiratory flow volume curves were obtained in all subjects. MZ subjects demonstrated statistically impaired maximal flow rates at 75%, 50%, and 25% of vital capacity compared to their MM controls. Total pulmonary resistance by the oscillometric method was measured at 3, 5, 7, and 9 cycle/s in the same subjects. Increased frequency dependence of resistance (defined as the difference between total pulmonary resistance at 3 cycle/s and 9 cycle/s) was observed in MZ subjects compared to MM controls. No differences were noted by this method in MS-MM pairs. The data suggest that detectable mechanical abnormalities are present in subjects with the MZ phenotype, even in the absence of established risk factors such as cigarette smoking and high air pollution.