The transforming growth factor-beta (TGF-beta) signaling pathway is a tumor-suppressor pathway that is commonly inactivated in colon cancer. TGF-beta is a secreted ligand that mediates its effects through a transmembrane heteromeric receptor complex, which consists of type I (TGFBR1) and type II subunits (TGFBR2). Approximately 30% of colon cancers carry TGFBR2 mutations, demonstrating that it is a common target for mutational inactivation in this cancer. To assess the functional role of TGFBR2 inactivation in the multistep progression sequence of colon cancer, we generated a mouse model that recapitulates two common genetic events observed in human colon cancer by mating Apc(1638N/wt) mice with mice that are null for Tgfbr2 in the intestinal epithelium, Villin-Cre;Tgfbr2(E2flx/E2flx) mice. In this model, we observed a dramatic increase in the number of intestinal adenocarcinomas in the Apc(1638N/wt);Villin-Cre;Tgfbr2(E2flx/E2flx) mice (called Apc(1638N/wt);Tgfbr2(IEKO)) compared with those mice with intact Tgfbr2 (Apc(1638N/wt);Tgfbr2(E2flx/E2flx)). Additionally, in vitro analyses of epithelial tumor cells derived from the Apc(1638N/wt);Tgfbr2(IEKO) mice showed enhanced expression and activity of matrix metalloproteinase MMP-2 and MMP-9, as well as increased TGF-beta1 secretion in the conditioned medium. Similarly, primary tumor tissues from the Apc(1638N/wt);Tgfbr2(IEKO) mice also showed elevated amounts of TGF-beta1 as well as higher MMP-2 activity in comparison with Apc(1638N/wt);Tgfbr2(E2flx/E2flx)-derived tumors. Thus, loss of TGFBR2 in intestinal epithelial cells promotes the invasion and malignant transformation of tumors initiated by Apc mutation, providing evidence that Wnt signaling deregulation and TGF-beta signaling inactivation cooperate to drive the initiation and progression, respectively, of intestinal cancers in vivo.