Mucin-producing tumors of the pancreas are rare and difficult to diagnose neoplasms. To further characterize the nature of these lesions, four such cases are described here in which the diagnosis was suspected at endoscopic retrograde cholangiopancreatography by the presence of ampullary or intraductal mucus. In one patient, pancreatoscopy directly visualized the tumor. Ultrasound and computed tomography failed to suggest the presence of a neoplasm in the majority of these cases, but endoscopic ultrasound was performed in one and correctly identified the multi-cystic tumor. A high incidence of acute pancreatitis was seen in these cases and was felt to be secondary to intermittent ductal obstruction by mucus. The characteristic appearance of intra-ductal mucus provides an important clue to the presence of this type of pancreatic neoplasia.