Progression of acute type B hepatitis to chronic liver disease and cirrhosis is well recognized, whereas no progression of acute type A hepatitis has as yet been documented. The natural history of acute non-A, non-B hepatitis has not been previously characterized. Ten cases of chronic liver disease were identified in 44 cases of acute non-A, non-B post-transfusion hepatitis. Age, sex, severity of acute illness, and prevalence of preoperative antibodies to known hepatitis-producing agents did not differ between the group whose hepatitis progressed to chronicity and the group whose hepatitis resolved. Less progression of acute hepatitis to chronic liver disease was seen in those patients receiving immune serum globulin preoperatively than in those receiving an albumin placebo (P = 0.009). Only 3 patients had clinical symptoms of hepatitis at the time of liver biopsy, and elevations of liver enzymes and gamma-globulin were mild. However, liver biopsy specimens in 8 of 10 patients showed chronic active hepatitis and an additional biopsy specimen showed cirrhosis. Acute non-A, non-B post-transfusion hepatitis often progresses to chronic active hepatitis. Preoperative gamma-globulin prophylaxis significantly reduces this progression. Identification and characterization of this viral agent(s) will further aid in the prevention of this undesirable complication of blood transfusion.