A 56-year-old man underwent distal pancreatectomy, splenectomy, and partial resection of the splenic flexure of the colon because of tumor in the tail of pancreas and the splenic hilus. The patient presented with symptoms of general malaise, anorexia, weight loss, mild diarrhea, and borderline diabetes mellitus, although there was no cholelithiasis. The diagnosis remained unclear until immunohistochemical studies of the resected specimen revealed somatostatin and synaptophysin, suggesting a somatostatinoma. Twenty-three reported cases of pancreatic somatostatinoma are reviewed and their clinical features discussed. The role of immunohistochemical studies in the diagnosis of somatostatinoma is described.