Heterozygous alpha 1-antichymotrypsin (ACT) deficiency is inherited in an autosomal dominant mode independently of alpha 1-antitrypsin with a gene frequency (q) of 0.003. In a previous study, a high prevalence of enlarged residual volumes in subjects with the trait were noted. Neutrophil cathepsin G, the target proteinase of ACT, enhances the elastolytic action of elastase. Thus, hypothetically, subjects with the trait may have increased risk for developing pulmonary emphysema. To test whether heterozygous ACT deficiency predisposes to lung disease, plasma ACT concentrations were determined in a cohort of 1,872 middle-aged women. Women with subnormal levels were studied with respect to heredity, airway symptoms, and lung function. Twelve women (0.64% of the cohort) were classified as heterozygotes after family studies and were compared with control subjects, matched for age, weight, sex, and smoking status. There were no significant differences in airway symptoms between heterozygotes and control subjects. However, the prevalence of ex-smokers was significantly higher among heterozygotes than among the screened population as a whole (prevalence ratio, 2.18; 95% confidence interval, 1.004-4.72). There were no differences between the heterozygotes and the control subjects in the basal spirometry. However, after bronchodilation, five of the 12 heterozygotes manifested residual volumes greater than 2.5 standard deviations above normal mean compared with one of 24 control subjects (p = 0.012). The present investigation thus confirms our previous findings of an increased prevalence of enlarged residual volumes in heterozygous ACT deficiency.(ABSTRACT TRUNCATED AT 250 WORDS)