We herein present two female siblings with persistent hyperinsulinemic hypoglycemia in the neonatal period who were diagnosed as having familial nesidioblastosis. Despite both the administration of diazoxide and the intravenous infusion of glucose, one of the affected infants died of severe metabolic acidosis at about 1 month of age, before pancreatectomy could be performed. The other, in whom the disorder was diagnosed early, also failed to respond to conservative medical treatment and ultimately required a 99% pancreatectomy for control of hypoglycemia. A third female sibling was normal. The possibility of familial nesidioblastosis should thus be considered in the case of neonatal intractable hypoglycemia to ensure a prompt diagnosis and allow for early surgical intervention when indicated. The relevant literature on this life-threatening disorder is also reviewed.