Background and Objective: To clarify the histogenesis of cancer of the gallbladder of the patients with pancreaticobiliary maljunction (PBMJ), the proliferating cell activity of the epithelium of gallbladder was examined. Methods: A total of 21 patients with pancreaticobiliary maljunction [8 with gallbladder cancer (MCA group) and 13 without cancer (M group)] were studied using immunohistochemical staining with a monoclonal antibody to the proliferating cell nuclear antigen (PCNA). The 23 gallbladder cancer patients without PBMJ (CA group) and 10 patients with normal gallbladders (N group) were also examined as controls. Results: The PCNA-positive rates of non-cancerous epithelium of the gallbladder in patients with PBMJ (14.2% in MCA group and 11.6% in M group) were significantly higher than those without PBMJ (3.9% in CA group and 1.5% in N group). Conclusion: The high proliferating cell activity of epithelium of the gallbladder may be an explanation for the high incidence of gallbladder cancer in patients with pancreaticobiliary maljunction. (C) 1997 Wiley-Liss, Inc.