Hyperinsulinism is the most common cause of hypoglycemia in early infancy. Congenital hyperinsulinism, formerly termed nesidioblastosis, is usually caused by genetic defects in beta-cell regulation, including a severe recessive disorder of the sulfonylurea receptor, a milder dominant form of hyperinsulinism, and a syndrome of hyperinsulinism plus hyperammonemia. Transient neonatal hyperinsulinism may be associated with perinatal asphyxia or small-for-dates birthweight and maternal diabetes. To prevent permanent brain damage from hypoglycemia, the treatment of infants with hyperinsulinism must be prompt and aggressive. A combination of medical therapy with diazoxide or octreotide, a long-acting somatostatin analog, and surgical 95% subtotal pancreatectomy may be required.