The production of interleukin (IL)-13 and unresponsiveness to IL-12 in T cells were analyzed in patients with active localized cutaneous leishmaniasis (untreated or not responsive to treatment) and in patients who had been treated successfully for the disease. More IL-13 was produced by specific T cells in response to Leishmania guyanensis (L. guyanensis) antigens in active compared to in inactive leishmaniasis. Furthermore, unresponsiveness of specific T cells to IL-12 was detected only in patients with active leishmaniasis, i.e. in patients with detectable parasites such as untreated patients and patients unresponsive to treatment. These results support that IL-12 unresponsiveness of Leishmania-specific T cells is responsible for the persistence of infection.