Giant-cell hepatitis is a frequent pattern of liver injury in the neonate, but it is rare after infancy. Such cases have been attributed to autoimmune disease, to non-A, non-B hepatitis and, most recently, to paramyxovirus infection. To better define the entity of postinfantile (syncytial) giant-cell hepatitis, we reviewed 24 biopsy specimens from 20 patients with this finding, either alone or in combination with other diagnoses. The number of multinucleated giant cells varied greatly from one specimen to another. Varying degrees of portal inflammation appeared in all but one of the patients, and all had hepatitislike acinar inflammation associated with hepatocellular injury. Fibrosis was a common finding, varying from mild periportal fibrosis to established cirrhosis (33%). The changes were interpreted as acute giant-cell hepatitis in 25%, as CAH in 42% and as active cirrhosis in the remainder. The patients ranged in age from 2 to 80 yr, with a mean of 35 yr and a male/female ratio of approximately 1:1. The signs and symptoms of liver disease were present for more than 1 mo in most patients. A positive antinuclear antibody titer was found in seven of the patients. Three patients had a direct Coombs reaction and anemia. Overall, evidence of autoimmune disease was found in 40% of the patients. One patient had non-Hodgkin's lymphoma involving the liver. Only one patient had a history of blood transfusion or risk factors for hepatitis C. No patient underwent serological study for paramyxovirus antibodies. Liver tissue from one patient was examined ultrastructurally, but no viral particles could be identified.(ABSTRACT TRUNCATED AT 250 WORDS)