We studied the risk of posttransfusion hepatitis in recipients of blood collected from volunteer donors who tested negative for HBsAg and had serum ALT levels less than 1.5 times the upper limit of the normal range. Between October, 1983 and September, 1984, 676 consecutive patients who needed blood or plasma transfusions during or after elective surgery, who had no history of liver disease and had never received blood previously, were studied. The patients were given a total of 4,813 (mean = 7) units. Ninety-six patients developed posttransfusion hepatitis, which yielded a hepatic incidence of 20 cases per 1,000 units of transfused blood. Ninety-two patients had non-A, non-B hepatitis, 3 had hepatitis B and 1 had cytomegalovirus infection. The incubation periods for non-A, non-B hepatitis ranged from 2 to 26 (mean = 9.5 +/- 4) weeks. In 68 (73%) patients, the hepatitis was completely asymptomatic; only 24 (27%) patients developed symptoms, including jaundice and hepatomegaly. There were no cases of fulminant hepatitis. Sixty per cent of the patients still had elevated serum ALT levels 1 year after the onset of hepatitis. The 96 patients with hepatitis had received a mean of 9.6 blood units, as compared to a mean of 6.7 units for the unaffected patients (p less than 0.001). This study demonstrated that non-A, non-B hepatitis remains a common and important complication of blood transfusion despite screening of blood donors for HBsAg and elevated serum ALT levels.