Sequential sera of 697 homosexual men, participating in a prospective study (1984-1986) of the risk to acquire human immunodeficiency virus (HIV) or AIDS, were tested for antibodies to human T-cell leukaemia virus (HTLV-I) by particle agglutination and immunoblotting. No intravenous drug users were included in this trial. Three men (0.4%) were HTLV-I antibody positive at intake and an additional 2 at the end of the observation period, resulting in an attack rate of approximately 0.3%. One of the 3 men with HTLV-I antibodies at intake was a Brazilian. One man had an acute HTLV-I infection after sexual intercourse with a Brazilian during holiday in Brazil. No serological cross-reactivity with HIV was observed nor a relationship with other sexually transmissible viral or bacterial infections. In contrast to HIV no relationship with anogenital intercourse was noted; both primary HTLV-I infected men practiced only orogenital intercourse. This suggests that HTLV-I was imported in the Dutch homosexual community after HIV was introduced in the Netherlands. HTLV-I appears to spread slower within the homosexual community than HIV and possibly by other routes.