Recent studies suggest that treatment of hemophiliacs with highly purified factor VIII concentrates may preserve immune function. To test this hypothesis, we prospectively studied 51 hemophilic patients (21 human immunodeficiency virus [HIV] seropositive and 30 seronegative) who were on home therapy exclusively with recombinant factor VIII (Kogenate, Miles Laboratory, Berkeley, CA) for 3.5 years. Patients, all of whom had been previously treated with plasma-derived factor VIII concentrates, were monitored every 6 months with T-lymphocyte subsets and β2-microglobulin levels. Mean rate of change in absolute CD4 cell counts, calculated from regression slopes for individual patients, showed a small but statistically significant decrease over the 3.5-year study period for HIV seropositive hemophiliacs. No decrease in CD4 cell counts was seen in HIV seronegative hemophiliacs when the data for children under age 6 years were excluded from the analysis. β2-microglobulin levels and CD8 cell counts remained unchanged. These data show stability of immunologic parameters in HIV seronegative hemophiliacs, and a small decrease in CD4 cell counts in HIV seropositive hemophiliacs treated with recombinant factor VIII.