Clinicopathological studies were carried out in 35 cases of mucin-producing tumor of the pancreas. In addition, natural history and serial pancreatogram changes were studied in 12 long-term follow-up cases. 83% of the main duct type and branch duct type greater than 4 cm were malignant, whereas hyperplasia cases were found in the branch duct type of 3 cm or less in size without marked dilation of the main duct and excrescent nodules. 33% of carcinoma cases were accompanied by parenchymal invasion and all those cases were of polypoid type of 2.5 cm or more in size. In long-term follow-up cases, all 3 cases of the main duct type showed progression of pancreatograms. In contrast, 8 of 9 cases of the branch duct type remained unchanged. These results suggest that surgical resection is mandatory in the main duct type and that the branch duct type where hyperplasia is highly suspected can be followed up.