STAT3 plays many roles in tumorigenesis. In this issue of Cancer Cell, Kortylewski et al. show that in the tumor microenvironment, STAT3 enhances the expression of the protumor cytokine IL-23 in macrophages but inhibits the antitumor cytokine IL-12 in dendritic cells. STAT3 also mediates IL-23's effect of activating tumor-infiltrating regulatory T cells.