To determine whether coinfection with HTLV-II influences the course of HIV-1 infection, we evaluated the progression from asymptomatic HIV infection (CDC group II) to persistent generalized lymphadenopathy (CDC group III) to AIDS-related complex (CDC group IVA) to full-blown AIDS (CDC group IVC) to death from AIDS in two groups of HIV-seropositive intravenous drug users (IVDUs). The first group consisted of 123 patients infected with HIV-1 only, and the second comprised 22 patients with serological and molecular evidence of HTLV-II/HIV-1 coinfection. Results of the immunological and clinical follow-up indicated a greater likelihood of developing persistent generalized lymphadenopathy among individuals infected with HIV-1 alone than among those coinfected with HTLV-II. However, no statistical difference was detected between the two groups in the depletion of CD4+ cells, the temporal decrease of the CD4/CD8 ratio, or the progression to ARC or AIDS or to death from AIDS. These findings suggest that HTLV-II may have no effect on the clinical evolution of HIV infection in IVDUs, which may be explained by the lack of pathogenicity of the HTLV-II coinfecting strain(s) and/or other still unclear biological or immunological cofactors or mechanisms.