Endoscopic ultrasonography (EUS) was performed in 23 patients with carcinoma of the ampulla of Vater (ampulla) and in 16 patients with common bile duct (CBD) carcinoma. These patients all underwent surgery. The layered structures of the duodenum, ampulla, and CBD, and the pancreas, portal vein, and regional lymph nodes were clearly identified by EUS using a transduodenal approach. With this technique, ampullary carcinoma appeared as a hypoechoic mass in 22 of 23 patients, and the 1 remaining cancer was not detected because of its small size. Carcinoma of the CBD also appeared as a hypoechoic mass in 12 of 16 patients. However, the remaining four appeared as hyperechoic masses. For these tumors, EUS had a high tumor detection rate (96 to 100%). In this regard, EUS was comparable to ERCP and was better than ultrasonography (US), CT, and angiography. Using EUS, we were also able to stage the extent of these tumors according to the involvement of the duodenal or CBD walls, invasion of the pancreas and portal vein, and spread to regional lymph nodes. The accuracy rates of cancer extent by EUS were 78% for ampullary carcinoma and 81% for CBD carcinoma when compared with surgical findings. We conclude that EUS is a valuable method for the detection and staging of tumors of the ampulla and CBD.