A case of multiple nonfunctional pancreatic islet cell tumor in multiple endocrine neoplasia type I (MEN I) is reported. The patient was a 41-year-old woman who had a past history of thyroid cancer (papillary carcinoma) and hyperparathyroidism due to parathyroid adenoma. Later, a nonfunctional pituitary tumor and five nonfunctional pancreatic tumors were found simultaneously and the patient was finally diagnosed as having MEN I. Following surgical enucleation, the pancreatic tumors were histopathologically diagnosed as benign islet cell tumors. One of them (tumor 3) exhibited a solid nodular pattern while the others showed gyriform patterns. They were divided histochemically and immunohistochemically into three types: two (tumors 1 and 2) produced a single hormone (glucagon), one (tumor 3) produced five (insulin, glucagon, somatostatin, gastrin and pancreatic polypeptide) and the remaining two (tumors 4 and 5) produced two (glucagon and pancreatic polypeptide). Electron microscopically, three types of endosecretory granules were found in the tumor cells of tumor 3 but only one type was found in tumor 4. However, in the tumor 4 extract, glucagon, pancreatic polypeptide, C-peptide, somatostatin, vasoactive intestinal peptide and growth hormone releasing factor were detected by radioimmunoassay. These findings suggest that these pancreatic tumors were both multicellular and multihormonal.