Pancreatic adenocarcinoma is the fifth leading cause of cancer death in the United States. Mutations in the K-ras oncogene occur in 85% of pancreatic adenocarcinomas and have also been identified in 75% of pancreatic ducts with mucinous cell hyperplasia seen in association with chronic pancreatitis. We identified K-ras mutations in 65% of duct lesions associated not only with chronic pancreatitis but also with pancreatic adenocarcinoma and distal common bile duct carcinoma (cholangiocarcinoma). These observations make K-ras a potential candidate for a gene-based diagnostic test. Indeed, K-ras mutations have been demonstrated in the pancreatic secretions of patients with pancreatic carcinoma and pancreatic intraductal neoplasia. We analyzed stool specimens for mutated K-ras sequences using a plaque hybridization assay in patients with pancreatic adenocarcinoma, cholangiocarcinoma, and chronic pancreatitis. K-ras mutations were detected in stool specimens from 6 of 11 patients with pancreatic adenocarcinoma, from 2 of 3 patients with cholangiocarcinoma, and from 1 of 3 patients with chronic pancreatitis. The K-ras mutations found in stool specimens from patients with pancreatic carcinoma were identical to those in the primary cancer in five cases. Mutations found in the stool specimens from one patient with pancreatic cancer, one patient with chronic pancreatitis, and two patients with cholangiocarcinoma were the same as those identified in pancreatic ductal mucinous cell hyperplasia lesions present in the resected pancreas specimens. Our data suggest that the K-ras mutations originating from cells of pancreatic adenocarcinomas and from cells shed by abnormal pancreatic duct epithelium can be detected in the stool. These results support the further exploration of stool K-ras analysis as a potential screening assay for the early detection of pancreatic adenocarcinoma and precursor lesions such as pancreatic ductal mucinous cell hyperplasia.