Intraductal mucin-hypersecreting neoplasms of the pancreas with extreme dilatation of the main duct were studied in eight patients. They included five men and three women, aged 47-85 years. Five patients had a history of symptoms mimicking pancreatitis; four developed steatorrhea and/or diabetes. At endoscopic retrograde pancreatography, five patients showed an open ampulla filled with mucin, and six patients showed patchy filling defects in the ectatic main duct. Morphological examination showed extreme dilatation of the entire pancreatic duct in six patients and its tail segment in two patients. The duct segments filled with viscous mucin were lined by well-differentiated mucin-secreting cells, forming papillary foldings and occasionally showing cellular atypia. None of the patients had invasive tumor or metastasis. Six patients whose lesions were resected are alive and doing well (mean follow-up, 5.5 years). It is concluded that intraductal mucin-hypersecreting neoplasm is a pancreatic tumor with favorable prognosis. Because it shares many features with intraductal papillary neoplasm, a common pathogenesis of these pancreatic tumors is suggested.