Anti–tumor necrosis factor-α (TNF-α) antibodies are frequently used to treat inflammatory diseases. However, these drugs also have immunosuppressive effects. We report on three patients who developed disseminated histoplasmosis on therapy with TNF-α inhibitors. In vitro assays were used to characterize the role of these agents in host defense against Histoplasma capsulatum. Intracellular proliferation of H. capsulatum was measured in alveolar macrophages and peripheral monocytes of normal volunteers in the presence and absence of the TNF-α antibody, infliximab. Both infliximab and control antibody enhanced fungal growth in monocytes and alveolar macrophages, suggesting this was a nonspecific antibody response. Despite similar intracellular fungal loads in the presence of both antibodies, lymphocyte proliferation in response to blood monocytes and alveolar macrophages infected with H. capsulatum was inhibited by the addition of physiologic doses of infliximab, whereas control antibody had no effect. The production of H. capsulatum–induced interferon-γ and TNF-α was assessed in 5-day cultures containing lymphocytes and alveolar macrophages or monocytes. Interferon-γ secretion was significantly reduced in the presence of infliximab. In summary, patients receiving anti–TNF-α therapy are at risk for developing disseminated histoplasmosis. This may be due to a defect in the TH1 arm of cellular immunity.