Congenital hyperinsulinism (CHI) is a disease phenotype characterized by increased, usually irregular, insulin secretion leading to hypoglycemia, coma, and severe brain damage, left untreated. Hyperinsulinism may be caused by a range of biochemical disturbances and molecular defects. In pancreatic β cells, insulin secretion is stimulated by closure of the ATP‐dependent potassium channel (K_ATP channel). K_ATP channel is a complex composed of at least two subunits: the sulfonylurea receptor SUR1 and Kir6.2, an inward rectifier K⁺ channel member. Mutations in both subunits have been identified in patients with the autosomal recessive form of hyperinsulinism, including 28 different mutations in the SUR1 gene and two mutations in the Kir6.2 gene. These mutations co‐segregated with disease phenotype, also known as persistent hyperinsulinemic hypoglycemia of infancy (PHHI), and with attenuated K_ATP channel function. Inadequately high insulin secretion in one family with an autosomal dominant mode of inheritance is caused by a mutation in the glucokinase gene, resulting in increased affinity of the enzyme for glucose. Five different mutations have been identified in the glutamate dehydrogenase gene, resulting in overactivity of this enzyme and causing a syndrome of hyperinsulinism and hyperammonemia. In 13 cases, hyperinsulinism was caused by one or more focal pancreatic lesions with specific loss of maternal alleles of the imprinted chromosome region 11p15. In five patients, this loss of heterozygosity unmasked a paternally inherited recessive SUR1 mutation. The new molecular approaches in PHHI give further insight into the mechanism of pancreatic β cell insulin secretion. The heterogeneous group of patients with CHI may now be classified according to their basic defects in the four different genes, with potential implications for a more specific treatment. Hum Mutat 13:351–361, 1999. © 1999 Wiley‐Liss, Inc.