Prostaglandin E₂ (PGE₂) is a bioactive lipid that mediates a wide range of physiologic effects and plays a central role in inflammation and cancer. PGE₂ is generated from arachidonic acid by the sequential actions of the COX and terminal synthases (PGES). Increased levels of COX-2, with a concomitant elevation of PGE₂, are often found in colorectal cancers (CRC), providing the rationale for the use of COX-2 inhibitors for chemoprevention. Despite their proven efficacy in cancer prevention, however, COX-2 inhibitors exhibit dose-dependent toxicities that are mediated in part by their nonspecific reduction of essential prostanoids, thus limiting their chemopreventive benefit. To achieve enhanced specificity, recent efforts have been directed toward targeting the inducible terminal synthase in the production of PGE₂, microsomal PGES (mPGES-1). In the present study, we show that genetic deletion of mPGES-1 affords significant protection against carcinogen-induced colon cancer. mPGES-1 gene deletion results in an about 80% decrease in tumor multiplicity and up to a 90% reduction in tumor load in the distal colon of azoxymethane (AOM)-treated mice. Associated with the striking cancer suppression, we have identified a critical role for PGE₂ in the control of immunoregulatory cell expansion (FoxP3-positive regulatory T cells) within the colon-draining mesenteric lymph nodes, providing a potential mechanism by which suppression of PGE₂ may protect against CRC. These results provide new insights into how PGE₂ controls antitumor immunity. Cancer Prev Res; 4(8); 1198–208. ©2011 AACR.