To determine if the manifestations of initial infection with Mycobacterium tuberculosis reflect changes in the balance of T cell cytokines, we evaluated cytokine production by M. tuberculosis- stimulated peripheral blood mononuclear cells (PBMCs) from 24 children with tuberculosis and 22 children who were healthy tuberculin reactors. PBMCs from patients with tuberculosis had lower production and mRNA expression of interferon γ (IFN-γ) than did PBMCs from healthy tuberculin reactors. IFN-γ production was most severely depressed in patients with moderately advanced and far-advanced pulmonary disease and in malnourished patients. Production of IL-12, IL-4, and IL- 10 was similar in tuberculosis patients and healthy tuberculin reactors. These results indicate that, during the initial immune response to M. tuberculosis, development of tuberculosis is associated with diminished IFN-γ production, which is not due to reduced production of IL-12 or enhanced production of IL-4 or IL-10.