Early reports suggested that hemophiliacs with factor IX deficiency (Christmas Disease) may be at less risk for developing the acquired immunodeficiency syndrome (AIDS) than patients with classic hemophilia. We evaluated 12 factor IX deficient patients for clinical and immunologic abnormalities related to infection with the human immunodeficiency virus (HIV). Antibody to HIV was not detected in these patients prior to 1982. By 1985, 66 percent (eight of 12) patients were seropositive. All three concentrates available commercially before 1985 were associated with seropositivity. Furthermore, seropositive hemophiliacs had received on average significantly more factor IX concentrate than seronegative hemophiliacs (27,825 +/- 17,976 (S.D.) versus 1,250 +/- 1,500 factor units/year, (p less than 0.02). Half of the seropositive individuals had generalized lymphadenopathy with splenomegaly. Two seropositive patients have developed AIDS, one with cryptococcal meningitis and another with a large cell immunoblastic lymphoma. Infection with HIV has occurred with high frequency in hemophiliacs who received unmodified factor IX concentrates.