A cohort of 180 haemophiliacs followed between 1983 and 1986 and a cohort of 961 homosexual men followed between 1984 and 1986 were compared for the prevalence and incidence of HIV-1 antibody (HIV-1-Ab) seropositivity, the incidence of AIDS-related complex (ARC) and AIDS and the prevalence and incidence of serological and immunological markers for HIV-related disease progression. Among the haemophiliacs 23 (12.8%) patients were HIV-1-Ab seropositive at entry and 20 (12.7%) of the remaining 157 seroconverted for HIV-1-Ab during follow-up. Of the homosexual men 238(24.8%) were HIV-1-Ab seropositive at entry and 68 (9.4%) of the 723 at entry seronegatives seroconverted during follow-up. Clinical follow-up of the HIV-l-Ab seropositive and seroconverted men was 59 months in the haemophiliac cohort and 60 months in the homosexual cohort. Among the HIV-l-Ab seropositive and seroconverted haemophiliacs and homosexual men the cumulative ARC/AIDS incidence was 2 and 18%, respectively. Occurrence of HIV-l-antigenaemia was more frequent among seropositive and seroconverted homosexual men (28%) than among haemophiliacs (7%) (p =0.001). The groups did not differ significantly for the absence or loss of anti-HIV core antibodies or the occurrence of low CD(4)+ cell numbers. These data indicate a slower progression of HIV-related disease in seropositive haemophiliacs compared to seropositive homosexual men.