Activation of the complement system, the main humoral mediator of inflammation, is restrained by the action of enzyme inhibitors including alpha 1-antitrypsin. Deficiency leads to chronic liver disease in about one in five children with this genetic defect. Complement activation was investigated in 34 children with alpha 1 AT deficiency (12 with minimal, 10 with moderate, and 12 with severe liver disease) and in 38 sex and age matched normal children by measuring the complement parent molecules C3, C4, the C3d fragment and by calculating the C3d:C3 ratio. C3 and C4 were lower in children with severe liver disease compared with controls, indicating impairment of hepatic protein synthesis or complement consumption. The C3d activation fragment was higher in all the patient groups when compared with controls while the C3d:C3 ratio, a measure of activation independent of the concentrations of the parent molecule, was higher in patients than in controls and increased with the degree of disease severity. These results suggest that complement may have a role in the pathogenesis of the chronic liver disease associated with alpha 1AT deficiency.