Intraductal papillary-mucinous tumor of the pancreas is a spectrum of conditions ranging from benign to malignant, and very few papers have referred to the natural history of this disease. In this communication the indicators of malignancy were examined from a viewpoint of natural history. Follow-up computed tomographies (CTs) more than 6 months after the diagnosis were reviewed in 17 Japanese patients with intraductal papillary-mucinous tumor of the pancreas. They were divided into two groups by the presence or absence of morphological progressive changes by the follow-up CTs, and the clinicopathological features were compared between the two groups to examine possible malignant indicators. The 17 patients consisted of seven patients in the no-change group and ten in the progressive group. The distribution of the patients was not different with regard to age; gender; or presence or absence of pancreatitis, diabetes mellitus, or unique findings of the ampulla of Vater between the two groups. The dilatation of the main pancreatic duct (> or = 3 mm) was more frequent in the progressive group: (eight of ten patients; 80%) than in the no-change group (two of seven patients; 29%) (P = 0.03). Six (86%) of the seven tumors in the no-change group were located in the branch duct, whereas five (50%) of the ten in the progressive group were situated in the main pancreatic duct. Histopathologic diagnoses of the resected specimens of the four in the no-change group examined were intraductal papillary-mucinous adenoma in three and adenoma with moderate dysplasia in one, whereas the diagnoses in the six in the progressive group examined were adenoma in two, adenoma with moderate dysplasia in two, and carcinoma (invasive) in two. The patients with intraductal papillary-mucinous tumor of the pancreas with a dilatation of the main pancreatic duct at the time of diagnosis should be followed up more carefully than those without dilatation. Once progressive morphological changes are detected by the follow-up CTs surgical resection should be considered because of possible malignancy.