This study examined the T cell responses in the early phase of Leishmania braziliensis infection. Cytokine profiles, lymphoproliferative responses, and skin test results in 25 patients with early cutaneous leishmaniasis (ECL; illness duration !60 days) were compared with those in persons with late cutaneous leishmaniasis (LCL; illness duration 12 months). Absent or low lymphoproliferative responses were observed in 8 (32%) of 25 patients and an absence of interferon (IFN)–g production in 9 (41%) of 22 patients prior to therapy. IFN-g production in ECL (mean 5 SD) was lower than in LCL (2935346 vs. 7475377 pg/mL, respectively; P ! .01). In contrast, interleukin (IL)–10 production in ECL (mean 5 SD) was higher than in LCL (246556 vs. 50541 pg/mL, respectively;P ! .01) .Restoration of lymphoproliferative responses and IFN-g production was achieved when monoclonal antibody to IL-10 or IL-12 was added to the cultures. These results show that T cell responses during early-phase infection are down-regulated by IL-10 and may facilitate parasite multiplication.