The immunological mechanisms underlying the susceptibility to disseminated visceral parasitism of mononuclear phagocytes in patients with kala-azar remain undefined. Resistance and susceptibility are correlated with distinct patterns of cytokine production in murine models of disseminated leishmanial disease. To assess lesional cytokine profiles in patients with kala-azar, bone marrow aspirates were analyzed using a quantitative reverse transcriptase PCR technique to amplify specific mRNA sequences of multiple Th1-, Th2-, and/or macrophage-associated cytokines. Transcript levels of IL-10 as well as IFN-gamma were significantly elevated in patients with active visceral leishmaniasis; IL-10 levels decreased markedly with resolution of disease. These findings suggest that IL-10, a potent, pleiotropic suppressor of all known microbicidal effector functions of macrophages, may contribute to the pathogenesis of kala-azar by inhibiting the cytokine-mediated activation of host macrophages that is necessary for the control of leishmanial infection.