We report a patient, a 23-year-old man, who had clinical and laboratory findings suggestive of insulinoma. Although imaging studies did not reveal any tumors in the pancreas, distal pancreatectomy was performed because the possibility of small insulinoma could not be completely excluded. Grossly, the surgically removed pancreas did not reveal any tumors. Microscopically, the pancreas exhibited islet cell hyperplasia and nesidioblastosis. To our knowledge, this is the first authentic reported case of islet-cell hyperplasia occurring in a Japanese adult.