Two cases of pancreatic F-cell malignant tumors, producing only pancreatic polypeptide (PP) with clinical, biochemical and immunohistochemical features are described. No PP-associated endocrine symptoms were present. In both tumors, definitive diagnosis was immunohistochemical, and surgical treatment was carried out. Clinical evolution in the two patients was different: one of them died within the postoperative period with upper gastrointestinal bleeding by multiple duodenal ulcers, whereas the other--with lymph node metastasis at intraoperative diagnosis--is alive six years after treatment. F-cell tumors can be classified among the group of silent endocrine tumors of the pancreas. The reason for their discrepancy in the clinical behavior is unknown.