Eleven of 27 haemophilic boys who received a common batch of Factor VIII concentrate subsequently developed acute hepatitis B; although 9 were considered not to have been previously exposed to the virus, 2 other boys had been considered immune to hepatitis B. The amount of concentrate received by each child, together with their HIV-antibody status and T-lymphocyte subset distribution prior to exposure, did not influence their response to the hepatitis B virus (HBV). The two previously immune children who became infected, however, had evidence of the HIV-associated persistent generalized lymphadenopathy syndrome. Detailed investigation of the suspect batch of concentrate revealed hepatitis B surface antibody to a titre of 112 miu/ml, but surface antigen was not detectable, even after dissociation of antigen and antibody. As a result of this outbreak, 5 of the 11 boys remain carriers of the virus and 2 other family members have contracted acute hepatitis B. The possibility that the response of the haemophiliacs to HBV may be altered due to acquired alteration of their immune function is discussed. Regular screening of haemophiliacs, including those immune to hepatitis B, is recommended, since even with regular donor screening, HBV remains a major infective risk to haemophiliacs receiving Factor VIII replacement therapy, and the risk to an individual may change with time.