Calcitonin release has rarely been reported in patients (pts) with neuroendocrine pancreatic tumors (NPT). The aim of this study was to describe the characteristics of calcitonin-secreting tumors (CST) of the pancreas. Serum calcitonin determination was part of the prospective evaluation of 66 pts with NPT referred to our institution over a 3-year period. Six pts (9%) had elevated calcitonin levels [at least twice the limit of the normal value (N)]. Abdominal ultrasonography, computed tomography scan, and endoscopic ultrasound were performed to identify the primary tumor(s) and metastases. Immunostaining using anticalcitonin and other antibodies was performed on the surgical resection specimen (four pts) or biopsy of liver metastases (two pts). Three of the six pts (four males, two females; median age, 51.5 years) had diarrhea. Serum calcitonin levels (median, range) were 17.5 N (6N-40N). Slight elevations in serum somatostatin (1.2N-2.3N) were associated in three pts. Pancreatic tumors were single in five of six pts and evenly distributed in the head and in the tail. Five pts had metastases, mainly in the liver. Multiple endocrine neoplasia type I was present in one pt. Immunostaining using calcitonin and somatostatin antibodies was positive in four pts each, respectively, and areas that were positive for one peptide were negative for the other. Diarrhea disappeared in the two pts who responded to treatment of the tumor(s). Three of the four pts with liver metastases died from tumor progression after 2, 10, and 24 months, respectively. CST of the pancreas are often malignant and can be considered as functional in half of the cases, irrespective of the serum calcitonin levels. Somatostatin secretion is often associated. Although rare, calcitonin secretion should be investigated in NPT pts presenting with diarrhea that cannot be explained by an increase in other hormone levels or in patients with nonfunctioning NPT.