To study the role of CD40 ligand (CD40L) in the host immune responses against intracellular pathogens, we infected CD40L knockout (CD40L^−/−) mice with Leishmania amazonensis. Although wild-type mice were susceptible to infection and developed progressive ulcerative lesions, tissue parasite burdens in CD40L^−/− mice were significantly higher. This heightened susceptibility to infection was associated with an impaired T cell and macrophage activation and altered inflammatory response, as reflected by low levels of IFNγ, lymphotoxin–tumor necrosis factor (LT–TNF), and nitric oxide (NO) production. Furthermore, CD40L^−/− mice failed to generate a protective immune response after immunization. These results indicate an essential role of cognate CD40–CD40L interactions in the generation of cellular immune responses against an intracellular parasite.