Pancreatic ductal adenocarcinoma (PDAC) is one of the most fatal human malignancies, with an overall 5-year survival rate of <5%. Genetic analysis of PDAC patient samples has shown that specific disease-associated mutations are correlated with histologically defined stages of neoplastic progression in the ductal epithelium. Activating mutations in KRAS are almost uniformly present in early-stage disease, with subsequent inactivating mutations in p16^INK4A, p53, and SMAD4 occurring in more advanced lesions. In this study, we have tested whether the loss of Smad4 would cooperate with an activating Kras^G12D mutation to promote progression to PDAC using the Pdx1-Cre transgenic system to activate Kras^G12D and delete Smad4 in all pancreatic lineages including the ductal epithelium. Analysis of double-mutant mice showed that loss of Smad4 significantly accelerated the progression of pancreatic intraepithelial neoplasias (mPanIN) and promoted a high incidence of intraductal papillary mucinous neoplasia and active fibrosis compared with Pdx1-Cre;Kras^G12D or Pdx1-Cre;Smad4^lox/lox mice. Occasionally, double-mutant mice progressed to locally invasive PDAC with little evidence of metastases by 6 months of age and without the detectable loss of p53 or p16^Ink4A expression or function. The loss of Smad4 only seemed to promote disease progression in the presence of the activated Kras^G12D allele because we observed no abnormal pathology within the pancreata of 23 Pdx1-Cre;Smad4^lox/lox animals that were analyzed up to 8 months of age. This indicates that Smad4 is dispensable for normal pancreatic development but is critical for at least partial suppression of multiple Kras^G12D-dependent disease-associated phenotypes. [Cancer Res 2007;67(17):8121–30]