The incidence of posttransfusion hepatitis and the rate of chronicity were investigated in a program devised at our hospital in December, 1982. Out of 2,596 blood recipients between January, 1982, and December, 1987, 451 (22.7%) developed posttransfusion hepatitis. Seventy-seven patients out of 217 (35.3%) whose course was closely followed progressed to chronicity. The incidence of posttransfusion hepatitis increased with the volume of transfused blood without any evident limitation. Recipients of elevated-ALT donor blood (greater than 26 Karmen units) were found to be more susceptible to posttransfusion hepatitis than those who had received only normal-ALT donor blood. Packed red blood cells, whole blood and fresh whole blood were high-risk components, and fresh frozen plasma a low-risk component of blood. The carrier rate of non-A, non-B hepatitis agents in Japanese healthy blood donors was determined to be 1.2% using the Frost-Reed model of infectious diseases. Anti-hepatitis C virus was detected in 62% of the cases of posttransfusion hepatitis 1 year after transfusion.