Intraductal ultrasonography (IDUS) was developed as a technique for visualizing arterial structures. We have employed IDUS during endoscopy to visualize the bile duct and pancreatic duct both in vitro and in vivo. We previously reported the clinical usefulness of IDUS in various pancreatic diseases, especially intraductal papillary mucinous neoplasm (IPMN) and pancreatic cancer. The IDUS probe usually used in recent years has a diameter of 6 French with a 20-MHz or 30-MHz radial scan transducer made by Aloka or Olympus. We inserted it into the biopsy channel of a duodenoscope and via the duodenal major papilla into the main pancreatic duct after endoscopic pancreatography without endoscopic sphincterotomy. In branch duct IPMN cases, over 90% of cases with mural nodules depicted by IDUS were carcinoma or adenoma. IDUS is useful for deciding whether surgical resection is indicated and determining the surgical resection line. In pancreatic cancer cases, IDUS can demonstrate a tumor as a hypoechoic lesion with irregular margins if the IDUS probe is led to the desired site. IDUS images of chronic pancreatitis cases show a rough pattern of pancreatic parenchyma with a hypoechoic band surrounding the main pancreatic duct corresponding to the periductal fibrosis occasionally. Furthermore, they do not have irregular margins like pancreatic cancer cases have. It is suggested that IDUS is useful for differential diagnosis between benign and malignant stricture of the main pancreatic duct. We encountered some cases in which only IDUS could demonstrate the tumor, which was not detected by any other modalities. IDUS, which makes it possible to evaluate pancreatic diseases, should be actively performed following endoscopic pancreatography.