The Veterans Administration has been conducting a cooperative randomized, double-blind, controlled trial to evaluate the efficacy of conventional and hepatitis B immune serum globulin for the prevention of post-transfusion hepatitis. Data collected between 1969 and 1974 provide the opportunity to describe the annual incidence and characteristics of the hepatitis that has developed, and the risk factors which have been identified. Anicteric hepatitis has developed four times more frequently than icteric hepatitis, the total incidence for all six years being 11.3 per cent. The incidence of HBs Ag-associated hepatitis declined dramatically after 1973 with the institution of routine screening of donor blood by radioimmunoassay techniques, although no change in the incidence of antigen-negative hepatitis has occurred. There is indirect evidence to suggest that an undefined agent is responsible for the majority of instances of post-transfusion hepatitis occurring presently. The most important risk factor responsible for the development of hepatitis is the use of commercial blood, and it is strongly urged that this form of blood be removed from general use.