BACKGROUND Although the biologic characteristics of papillary cystic tumors (PCT) generally indicate a good prognosis, a malignant form has been reported. METHODS Twenty-two examples of PCT were examined to assess their malignant potential by histologic, flow cytometric, and immunohistochemical studies. RESULTS Three had hepatic, peritoneal, and/or lymph node metastases (metastasizing tumors [MT]); the other 19 PCT had no such metastatic features (nonmetastasizing tumors [NMT]). Venous invasion was detected in all three MT and two NMT. Compared with the 19 NMT, the MT had a higher nuclear grade and more prominent necrobiotic nests characterized by aggregates of cells with pyknotic nuclei and eosinophilic cytoplasm. In the flow cytometric analysis of cellular DNA content, one MT was aneuploid, and eight NMT were diploid. Immunohistochemically, there was no difference between the MT and NMT. CONCLUSIONS These results support the assumption that venous invasion, nuclear grade, and prominent necrobiotic nests are useful as histologic parameters to detect the malignant potential of PCT.