It is supposed that Leishmania infection increaseshuman immunodeficiency virus (HIV) replication in seropositive individuals. Two groups of 9 HIV-infected intravenous drug users each, one group with HIV-Leishmania coin-fection (as determined by bone marrow microscopy, culture and an immunofluorescent assay, the other with HIV infection alone, but no evidence of Leishmania coinfection were matched for sex, age, time since first diagnosis of HIV infection, number of AIDS-defining diseases, proportion of patients treated with AZT and months of treatment, CD4/CD8 ratio, β₂-microglobulin level and HIV p24 antigen positivity rate. IL-4, -6, -10 and -12 and IFN-γ levels were determined by commercial enzyme immunoassays. The HIV-1 RNA copy number was quantified with the nucleic-acid-sequence-based amplification method (NASBA). The differences between the two groups were highly significant for all markers determined except for IL-12 and IFN-γ. We found a higher viral load in the patients with HIV’-Leishmania coinfection compared to the patients with HIV infection alone (p < 0.009). In 6 HIV-positive individuals without Leishmania coinfection, the HIV-1 RNA copy number was below the detection limit of NASBA (i. e. < 400 copies/100 µl). Plasma levels of IL-4, -6, and -10 were significantly elevated in the coinfected group (p < 0.0001; p < 0.02, and p < 0.005). The results of our study show that the viral load is increased in patients with HIV-Leishmania coinfection in comparison to the controls. This might be partly due to Th2 immune activation, as demonstrated by higher plasma levels of IL-4, -6 and -10 in HIV-Leishmania-coinfected patients than in HIV-infected individuals.