Allogenic proteins that contaminate intermediate purity clotting factor concentrates may activate the immune system of HIV-infected haemophilic patients. In 37 haemophilic patients infected with HIV who had originally been treated with intermediate purity factor VIII concentrate and then changed to monoclonally-purified high purity factor VIII concentrate the rates of CD4+ decline (10(9)/l per year) were 0.06 before and 0.02 after a switch to high purity products (p = 0.01). The median follow-up of patients after the switch to high purity products was 1.7 years (range 0.2 to 3 years). This significant change in the rate of CD4 decline was independent of the starting CD4 count, age and antiretroviral therapy. This result is consistent with those from randomised trials of the introduction of high-purity concentrate. Given the strong association between the CD4+ count and survival, treatment with high purity rather than intermediate purity clotting factor concentrate may confer a survival benefit for HIV-infected haemophilic patients.