A 50-year-old Korean man with repeated episodes of temporary loss of consciousness was diagnosed as having hyperinsulinemic hypoglycemia. Under the tentative diagnosis of insulinoma, localization procedures were carried out but no tumor was found. By percutaneous transhepatic portal venous sampling, no definite gradient in insulin concentration was found. During exploratory laparotomy no tumor was palpable in the pancreas, and intraoperative ultrasonography showed low echogenicity in the pancreatic head. Whipple's operation was performed and 70% of the proximal pancreas was removed. Histomorphometric examination of the resected specimen revealed graded hyperplasia of the islet cells. The most profuse hyperplasia was noted in the head with progressive decrease in the degree of hyperplasia to the body and tail. The patient remains euglycemic and tolerates 24 h fasting without any medication until 15 months after operation.