We evaluated 224 children from Rochester, New York, families in which at least one parent was heterozygous for alpha-1-antitrypsin (AAT) deficiency by protease inhibitor (Pi) typing. The childhood population included, of the two major heterozygote Pi types, 75 Pi type MS children and 37 Pi type MZ children. This population was evaluated by means of a standardized respiratory questionnaire, physical examination, and pulmonary function tests. Maximal expiratory flow volume curves were obtained with subjects breathing both air and a helium-oxygen mixture. Total pulmonary resistance by the method of forced oscillations was measured at 3, 5, 7, and 9 cps. No differences in respiratory symptoms or physical findings were seen in Pi type MZ and MS children compared to Pi type MM children. However, when pulmonary function tests were evaluated by a matched-pair analysis designed to minimize other genetic and environmental risk factors, Pi type MZ subjects demonstrated statistically significant differences in forced expiratory flow rates and in increased frequency-dependent characteristics of total pulmonary resistance. These abnormalities are similar to those previously described in adult Pi type MZ subjects, and suggest that physiologic abnormalities associated with AAT deficiency are present early in life.