This study aimed at evaluation of the immunological status and the clinical course of both HIV seronegative and seropositive hemophiliacs treated with either an ultra-pure factor VIII product (UP-F VIII), or a high-purity F VIII (HP-F VIII) concentrate. Eighteen HIV seronegative patients were divided into two groups of therapy and their immune status was followed for 2 years. During the second year of the study 8 patients of the HP-F VIII and 6 from the UP-F VIII therapy groups were switched to the alternative F VIII concentrates. Eighteen asymptomatic HIV seropositive patients were also divided into therapy groups and their immune status and any development of HIV-related symptoms were followed for 4 years. Evaluation of the HIV seronegative patients during the first year did not reveal any differences between the groups in the CD4 or CD8 cell counts, in natural killer cell (NK) activity, or in the mitogenic responses of T lymphocytes to Phytohemagglutinin (PHA), and of B lymphocytes to Pokeweed mitogen (PWM). The switch of 8 patients from the HP-F VIII and 6 from the UP-F VIII groups to the alternative concentrate did not yield any changes in their immune profile during the second year of the study. The HIV seropositive groups differed in the initial CD4 count, with a lower CD4 count (193 ± 126 vs 437 ± 142) and a higher F VIII consumption (63,000 ± 17,000 vs 26,000 ± 10,000) in the UP-F VIII group. During four years of follow-up the annual decline of CD4 counts in the UP-F VIII group was 8 (5%) compared with 74 (17%) in the HP-F VIII group, with only the latter being significant. The decrease in delayed type hypersensitivity, was also significant in the HP-F VIII group alone. Cumulative HIV-related symptoms with an advancement from CDC category II to category IV were observed in 2 and 5 of the UP-F VIII and HP-F VIII treatment groups, respectively. Our data, taken separately or together with previously published studies, demonstrate the impact of purity of the F VIII concentrate on CD4 decline and possibly also on the clinical course in HIV seropositive hemophiliacs. In the seronegative group, we found no support for a beneficial effect on the immune system by switching to an ultra-pure F VIII concentrate.