Of 120 patients who were investigated for moderately elevated liver function tests as the only sign of liver disease, 6 young persons had alpha-1-antitrypsin (AAT) deficiency. Three had a homozygous (Pi ZZ) and three had a heterozygous (Pi MZ) AAT deficiency as measured with isoelectric focusing. An extensive investigation ruled out all other causes of liver disease. The three homozygous patients showed typical periodic acid-Schiff (PAS)-positive globules in their liver biopsies and slight fibrosis, whereas none of the heterozygous patients showed these features. Electron microscopical investigation also showed typical findings in the homozygous but not in the heterozygous patients. Further development of liver disease in these young and apparently healthy AAT-deficient patients with early signs of liver damage is not known. It is possible that these patients will develop severe liver disease later in life. It was possible to detect only the three homozygous patients by histochemical examination of liver tissue, since the heterozygous patients did not show PAS-positive globules in their liver.