Fifteen cases of papillary cystic tumor of the pancreas (PCTP) were studied (14 female patients, one male patient; mean age: 23.5 years). Most tumors developed in the head of the pancreas as a well circumscribed large mass. The tumor had a mean diameter of 6.7 cm(range; 2 to 15 cm). Histopathologically abundant delicate papillary fragments, monomorphic tumor cells and degenerative changes of the solid area of the tumor were characteristic. All but two cases had completely circumscribed capsules. Two cases had duodenal invasion; one of all cases had cul de sac metastasis. Compared with 12 non-aggressive tumors, the aggressive cases had larger tumor size (more than 9 cm) with a thicker capsule (more than 2 mm). In studies to investigate the prognostic index using nucleolar organizing region (NOR), proliferating cell nuclear antigen (PCNA) and flow cytometry as well as nuclear grade and mitotic index, we could not find the useful parameter to detect the malignant potential of PCTP. In the flow cytometric analysis of cellular DNA contents, two invasive cases and the only one case of the male patient among the non-aggressive group were aneuploid. In conclusion, although it is hard to predict the prognosis by microscopic findings only, those with a thick capsule and aneuploidy tend to be related to malignant potential.