The most common cause of hyperinsulinemic hypoglycemia in adults is insulinoma. Nesidioblastosis is a rare, but well-recognized disorder of persistent hyperinsulinemic hypoglycemia in infancy, but adult-onset nesidioblastosis associated with hyperinsulinemic hypoglycemia, termed noninsulinoma pancreatogenous hypoglycemic syndrome (NIPHS), has been reported. Here, we describe an extremely rare case of NIPHS in an elderly man. A 78-year-old man was admitted to our hospital due to hypoglycemic coma. During the previous 3 months, he noticed excessive sweating at midafternoon. His low fasting plasma glucose level (27 mg/dl) and high immunoreactive insulin level (11.1 muU/ml) were consistent with the possible presence of insulinoma. Localizing studies including computed tomography of the abdomen and celiac arteriography were negative, but selective arterial calcium infusion (SACI) test suggested the presence of insulinoma in the body and tail of the pancreas. Surgical exploration by palpation and intraoperative ultrasonography failed to detect any mass in the pancreas, and 60% distal pancreatectomy was performed. Postoperatively, his hypoglycemic episodes completely disappeared. Histological examination of the resected pancreas revealed diffuse islet cell hyperplasia consistent with a pathological diagnosis of nesidioblastosis. Thus, our case is a very rare case of NIPHS, or adult-onset nesidioblastosis, in which SACI test was proven to be a useful diagnostic tool for localization of the pancreatic lesion.