Leishmania parasites persist in their vertebrate host after the treatment-induced clinical cure and in the asymptomatic infection. They confer resistance to reinfection but represent a risk of occurrence of acute leishmaniosis in immunosuppressed conditions. We examined the effects of prolonged dexamethasone administration on a chronic Leishmania infantum infection. Splenic T cell populations from the long-term-infected BALB/c mice were reduced by 55%, whereas those from uninfected controls were depleted by 85%. The ability of the remaining spleen cells to produce IL-2, IFN-gamma, IL-4 and TNF-alpha after in vitro specific stimulation decreased twofold, and the specific anti-leishmanial antibodies declined 3- to 5-fold. Liver, spleen and bone marrow are the main L. infantum targets in natural and experimental infections. Three-fold increase of amastigote burden was evidenced in the spleen, after dexamethasone administration was prolonged for over 2 months. No reactivation of Leishmania proliferation was disclosed in the liver and bone marrow. These results show a decreased sensitivity of splenic T cells to dexamethasone in a chronic Leishmania infection and a distinct response of the Leishmania-infected target organs to the dexamethasone-induced immunosuppression.