Immunity to infection with intracellular pathogens is regulated by interleukin 12 (IL-12), which mediates protective T helper type 1 (T_H1) responses, or IL-4, which induces T_H2 cells and susceptibility. Paradoxically, we show here that when present during the initial activation of dendritic cells (DCs) by infectious agents, IL-4 instructed DCs to produce IL-12 and promote T_H1 development. This T_H1 response established resistance to Leishmania major in susceptible BALB/c mice. When present later, during the period of T cell priming, IL-4 induced T_H2 differentiation and progressive leishmaniasis in resistant mice. Because immune responses developed via the consecutive activation of DCs and then T cells, the contrasting effects of IL-4 on DC development and T cell differentiation led to immune responses that had opposing functional phenotypes.