We have done endoscopic retrograde pancreatography-aspiration cytology for 81 consecutive patients in whom there had been neither pancreatic mass nor duct stenosis; cancer cells were detected in four cases (5%). This report reviews a new method of intraoperative cytodiagnosis, which was done for these four cases, to locate the original lesion of the cancer cells. This method involved dividing the pancreatic neck at a right angle to the main pancreatic duct. A catheter was then inserted in both the caudal and cranial ducts of Wirsung. With the aid of an intravenous injection of secretin, pure pancreatic secretions were collected separately and used for intraoperative cytodiagnosis. With this method, all four occult neoplasms of the pancreas were correctly identified as to whether they were located in the cranial, caudal, or both portions. In one case in which carcinoma in situ extended throughout the entire pancreas, total pancreatectomy had been indicated by the positive results obtained from both portions. Likewise, in the other three cases in which, the occult neoplasms were limited to the cranial or caudal pancreas, either a Whipple procedure (one case) or a caudal pancreatectomy (two cases) had been quite correctly indicated. Since this method is simple, safe, and reliable, it is beneficial to the patient with occult neoplasm, because it prevents blind resection of the entire pancreas without any fear of leaving the neoplastic lesion behind.