Clinicopathological features of 28 patients with intraductal papillary tumor (IDPT) and 10 patients with mucinous cystic tumor (MCT) of the pancreas were studied. Both IDPT and MCT showed papillary projections of the epithelium with abundant mucus secretion in the ectatic ducts or cystic spaces. The patients with IDPT comprised 19 men and 9 women with a mean age of 64.9 years. Three fourths of the IDPTs were located in the pancreatic head, and the mean tumor size was 3.5 cm. Local recurrence was observed in one patient, but none died of IDPT. In contrast, all patients with MCT were women, with a mean age of 49.4 years. None of the MCTs arose in the head, and the mean tumor size was 7.1 cm. One patient died of MCT, but all of the others survived without recurrence. Eight IDPTs and three MCTs showed invasion into the surrounding pancreatic tissue. Muconodular infiltration was mainly observed in five IDPTs and one MCTs and tubular infiltration in three IDPTs and two MCTs. A characteristic histological finding associated with the muconodular infiltration in IDPT was subepithelial "mucin droplets" that appeared to represent a change in polarity of mucus secretion. The formation of such subepithelial "mucin droplets" may be the initial step of muconodular infiltration in IDPT. Muconodular infiltration in IDPT appears different morphologically and biologically from the mucinous carcinoma subtype of conventional invasive ductal carcinoma.