A 75-year-old woman had an operation for gallstone ileus without cholecystectomy in other hospital and she was admitted to our hospital because of duodenal adenoma with severe atypia and small carcinoid in proximal duodenal wall. Distal gastrectomy and cholecystectomy were performed. Histological studies revealed the existence of cholecystoduodenal fistula and suggested the existence of gallbladder carcinoma progressed to the duodenal wall through the fistula. Cystic duct dissection and lymph nodes dissection were performed. It has been theorized that a cholecystoduodenal fistula may represent a significant risk factor in the development of gallbladder carcinoma because of the chronic reflux of duodenal contents which includes pancreatic juice. Our case may support this theory. In this case, we thought that the formation of gallbladder cancer could have been avoided if the cholecystectomy was performed in the first operation for gallstone ileus. It is very important that cholecystectomy should be performed when an existence of cholecystoduodenal fistula is highly suspected.