Paracoccidioidomycosis, the major systemic mycosis in Latin America, is caused by the thermally dimorphic fungus Paracoccidioides brasiliensis. To investigate the role of interleukin (IL)-12 in this disease, IL-12p40^-/- deficient mice (IL-12p40^-/-) and wild type mice (WT) were infected intravenously with viable yeast cells of P. brasiliensis 18 isolate. We found that, unlike WT mice, IL-12p40^-/- mice did not control fungal proliferation and dissemination and succumbed to infection by day 21 after inoculation. Additionally, IL-12p40^-/- mice presented a higher number of granulomas/mm² in lung tissue than WT mice, and showed unorganized granulomas containing high numbers of yeast cells. Moreover, IL-12p40^-/- mice did not release detectable levels of IFN-γ, but they produced high levels of IL-10, as well as IgG1 antibody. Additionally, splenocytes from both infected IL-12p40^-/- and WT mice exhibited a suppressed Con-A-induced T cell proliferative response. Our findings suggest that the IL-12p40 subunit mediates resistance in paracoccidioidomycosis by inducting IFN-γ production and a Th1 immune response