One-hundred-and-ten children between the ages of two months and 14 years with the following liver diseases were studied: 16 with acute viral hepatitis, 8 with persistent chronic hepatitis, 31 with active chronic hepatitis, 5 with hepatic steatosis, 11 with cirrhosis of the liver, 24 with newborn cholestasis, 3 with Wilson's disease, 2 with congenital hepatic fibrosis, 5 with metabolic diseases and 5 due to other causes. These children presented Pi system phenotypes in isoelectric focus using ultrafine polyacrylamide gels according to Kuepper's method, with modifications incorporated to determine Alpha-1-antitrypsin (A1-AT) serum level deficiencies in those presenting the Pi ZZ phenotype, a liver biopsy with P.A.S. coloration on digestion of diastase and a family history of the phenotype. Four (3.6%) of the children with Pi ZZ phenotypes showed a decrease of serum A1-AT and the presence of positive P.A.S. inclusions resistant to diastase in the cytoplasm of hepatocytes. Three had a history of postnatal icterus and the fourth presented hepatomegaly. The phenotypic study of the parents showed their being heterozygous (MZ), while siblings were normal (MM). The importance of the diagnosis of A1-AT deficiency and the diagnostic value of detecting Pi system phenotypes in every case of liver disease in children and adolescents is stressed.