We coincubated killed or live human immunodeficiency virus type-1 (HIV-1) with human monocyte-derived cells infected with Leishmania donovani and examined the effect of the virus preparations on the intracellular growth of the parasite. We found that there was significant enhancement (by a mean of 53%, p < 0.001) of intracellular L. donovani growth in the human monocytic leukaemia THP-1 cell line coincubated with killed HIV-1. Infection of peripheral blood monocyte-derived macrophages with live HIV-1 initiated after L. donovani infection led to an increase in intracellular parasites by an overall mean of 2.8% vs 4.9% (p < 0.01) at 2 and 5 d after HIV infection in L. donovani and L. donovani plus HIV-1 infected, respectively, and by an overall mean of 5.0% vs 13.3% (p < 0.001) at 5, 12 and 15 d after HIV-1 infection in L. donovani and L. donovani + HIV-1 infected, respectively. Further, L. donovani infection 2 d after infection with HIV-1 led to enhanced parasite growth (34.5%, p < 0.001) compared with cells infected with L. donovani alone (5.5%), and those where HIV-1 was added after L. donovani (18.1%). In all cases, HIV-1 from live and killed virus preparations led to decreased anti-leishmanial activity of the macrophages as evidenced by decreased control of intracellular multiplication. The findings may suggest a mechanism not requiring live virus to explain how HIV-1 coinfection may impair the control of intracellular Leishmania growth in individuals with pre-existing asymptomatic infection leading to the reactivation of the parasite. Moreover, patients with HIV-1 infection might be at increased risk of developing Leishmania infection.