A group of 163 patients with chronic obstructive pulmonary disease, from the pulmonary service of a large urban hospital, were evaluated for their protease inhibitor (Pi) type by starch gel and crossed immunoelectrophoresis, for serum concentrations of α₁-antitrypsin and α₁-antichymotrypsin, and for pulmonary function. Of the patients with emphysema, 17.8 per cent were of Pi type Z; 50 per cent of these were less than 45 years of age, compared to 13 per cent of those of Pi type M. Of all patients with chronic obstructive pulmonary disease, 4.9 per cent were of Pi type Z; 4.9 per cent of patients were of Pi type MZ (heterozygotes) compared with 1.9 per cent of the control population. There was an increased incidence of chronic obstructive pulmonary disease in persons of Pi type MZ, but no increase in persons of Pi type MS. Concentrations of both α₁-antitrypsin and α₁-antichymotrypsin were increased and were correlated. No patient had a deficiency of α₁-antichymotrypsin.