The frequency of occurrence of alpha-1-antitrypsin (A1AT) deficiency among total of 3228 Polish children with chronic liver diseases and chronic disease of respiratory tract was determined. It was observed that among children with chronic liver diseases which disclosed more frequent defect (concentration of A1AT below 150 mg/dl was found in 10.3% of children), the highest occurrence of deficiency was in children with neonatal hepatitis (23.1%). The deficiency was connected with the presence of ZZ and MZ phenotypes of A1AT.