In a prospective study of street-recruited intravenous drug users (IVDUs) in Miami, Florida, USA, 107 (46.5%) of 230 IVDUs were infected with HIV-1. Of these HIV-1 seropositive individuals, 23 were also infected with human T-lymphotropic virus type I or II (HTLV-I/II). To test the hypothesis that HTLV-I/II infection has an effect on the survival of HIV-1 seropositive IVDUs, various baseline clinical and laboratory HIV-1 related indices in HTLV-I/II positive and negative groups were compared. Life table analysis and Cox's proportional hazards model were used to estimate the potential effect of HTLV-I/II infection on the survival patterns of people infected with HIV-1. IVDUs infected with both viruses were three times more likely to die from AIDS during follow-up than were those infected with HIV-1 only. This finding suggests that HTLV-I/II seropositivity may adversely affect the clinical outcome of HIV-1 seropositive patients.