To assess the relationship between autoimmune pancreatitis (AIP) and pancreatic cancer, we analyzed K-ras mutation in the pancreatobiliary tissues of patients with AIP.An analysis of K-ras mutation and an immunohistochemical study were performed on the pancreas of 8 patients with AIP and 10 patients with chronic alcoholic pancreatitis and on the common bile duct and the gallbladder of 9 patients with AIP. K-ras mutation was analyzed in the pure pancreatic juice from 3 patients with AIP.High-frequency K-ras mutation (2+ or 3+) was detected in the pancreas of all the 8 patients and in the pancreatic juice of the other 2 patients. The mutation in codon 12 of the ras gene was GAT in all the 10 patients. High-frequency K-ras mutation was detected in the common bile duct of 5 patients with AIP and in the gallbladder epithelium of 4 patients with AIP. The K-ras mutation was detected in the fibroinflammatory pancreas, the bile duct, and the gallbladder, with abundant infiltrating IgG4-positive plasma and Foxp3-positive cells of patients with AIP with elevated serum IgG4 levels.Significant K-ras mutation occurs most frequently in the pancreatobiliary regions of patients with AIP. Autoimmune pancreatitis may be a risk factor of pancreatobiliary cancer.