The clinical features and changes of the pancreatic duct system in nine patients with mucin-producing tumor of the pancreas were serially studied to clarify the natural history and evolution of pancreatograms. The observation periods ranged from 6 to 50 months (mean, 30 months), and all patients were examined by endoscopic retrograde pancreatography two or more times. Four of the nine cases exhibited obvious progression of pancreatic duct dilation in association with increased amounts of mucin in the dilated duct, whereas five did not show apparent progression of ductal changes. Changes in the pancreatograms of mucin-producing tumor occurred when mucin secretion by the tumor became prominent, and such cases frequently presented with abdominal pain and pancreatitis. In contrast, most of the branch duct type remained unchanged; all three cases of main duct type showed progression of pancreatograms. There was no apparent relationship between degree and progression of ductal changes and the tumor malignancy.