A 66-year-old woman with a 10-year-history of diabetes mellitus was admitted to our hospital for investigation of several recent attacks of hypoglycemia. Her fasting blood glucose level was very low, at 30-40 mg/dl, and abdominal ultrasonography and computed tomography revealed a tumor in the pancreatic tail with fatty changes. Endoscopic retrograde cholangiopancreatography revealed absence of the main pancreatic duct from the body to tail of the pancreas. Abdominal angiography showed a hypervascular tumor stain in the pancreas, and percutaneous transhepatic portal vein sampling demonstrated a step-up of immunoreactive insulin levels in the splenic vein. Based on these clinical findings, we made a preoperative diagnosis of an insulinoma accompanied by fatty changes in the pancreatic body and tail. During laparotomy for the insulinoma, fat tissue was identified in the anatomic location of the pancreatic body and tail, and resected. Pathological examination of the resected specimen revealed a number of Langerhans islets in the adipose tissue, and an islet cell tumor with fatty replacement of the pancreatic tissue around the tumor. The insulinoma was found not to have caused obstruction of the main pancreatic duct. We present herein a rare case of an insulinoma that developed in the pancreas, and was associated with fatty replacement of unknown etiology.