Viral hepatitis is still the most serious post-transfusion complication. Despite the routine screening of donor blood for the hepatitis type B surface antigen in the United States, post-transfusion hepatitis develops in approximately 7% of blood recipients. Type B hepatitis accounts for only 10% to 15% of cases; non-A, non-B hepatitis constitutes the remainder. Non-A, non-B hepatitis is usually asymptomatic and anicteric but often runs a prolonged course manifested by persistent or intermittent elevations of alanine aminotransferase levels. The risk of developing non-A, non-B hepatitis is increased considerably when blood from paid rather than volunteer donors is transfused. Although there are currently no definitive preventive measures that can be used to reduce the attack rate, the possibility of developing a serologic test fot the non-A, non-B agent is under active investigation.