To determine the prevalence of alpha 1-antitrypsin deficiency in patients with cirrhosis or chronic active hepatitis, we performed a five-year prospective study of liver-biopsy specimens from 1055 adults. Thirty-four patients whose specimens contained hepatocyte inclusions characteristic of the deficiency were phenotyped, and 25 had phenotype MZ (2.4 per cent). The distribution of patients with this phenotype among the 185 patients with cirrhosis diagnosed histologically was three of 84 patients with alcoholic cirrhosis (3.5 per cent), seven of 34 with non-B chronic active hepatitis (20.5 per cent), six of 28 with cryptogenic cirrhosis (21 per cent), and one of 39 with other kinds of cirrhosis (2.6 per cent). The increased prevalence of MZ in patients with cryptogenic cirrhosis and with chronic active hepatitis is highly significant (P < 0.001). Because serum levels of alpha 1-antitrypsin may be unreliable for identification of the subgroup of patients with chronic active hepatitis or cryptogenic cirrhosis, analysis of serum for the MZ phenotype and meticulous examination of biopsy specimens may be necessary.