We report herein the clinicopathological, immunohistochemical and ultrastructural studies on 13 female patients and one male patient with papillary cystic tumors of the pancreas. Their ages ranged from 12 to 60 (mean 25) years. Most patients complained of abdominal mass or abdominal pain. Following complete resection of the tumor, all have remained well for between 3 months and 19 years (maen 5 years). In one patient the tumor was malignant and, 10 years after the initial partial resection, there was a recurrence with involvement of the colon, metastasis to the lymph nodes and venous invasion. Immunohistochemically, most tumor cells were positive for neuron specific enolase, synaptophysin, alpha-1-antitrypsin and vimentin and sometimes for the estrogen receptor related antigen, ER-D5, and the oncogen product of c-Ha-ras, c-Ha-ras P21. Ultrastructurally there were zymogen-like intracytoplasmic granules, intercellular junctions and intercellular spaces. These results support the hypothesis that the tumor originates from undifferentiated cells capable of differentiation toward acinar, endocrine or ductular cells. Estrogen and the c-Ha-ras oncogene presumably are linked to the development.