Six hemophilia patients previously seronegative for human immunodeficiency virus (HIV) seroconverted between September 1986 and September 1987. None had risk factors for HIV infection other than hemophilia. We compared the factor concentrates received by these patients with the concentrates received by 10 seronegative hemophilia patients. A statistically significant association was observed between seropositivity and the receipt of two lots of Factor VIII produced from the same plasma pool (odds ratio 77, p = 0.0014); five of the six case subjects but none of the control subjects had received concentrate from one of the two lots. Available evidence suggests that the sixth case subject had also received concentrate from an implicated lot. Symptoms including rash and fever were reported in five cases within 6 weeks after the implicated concentrate had been given. The implicated lots were produced from plasma from paid donors that had been screened and then heated at 60 degrees C for 30 hours in the lyophilized state. Subsequent to our investigation all concentrate produced by this process was removed from distribution.