Closure of ATP-sensitive potassium channels in pancreatic islet beta-cells initiates a cascade of events that leads to insulin secretion. beta-Cell ATP-sensitive potassium currents can be reconstituted by coexpression of the inward rectifier Kir6.2 and the sulfonylurea receptor (SUR), a member of the ATP-binding cassette superfamily. Mutations in SUR have been identified in individuals affected with familial persistent hyper-insulinemic hypoglycemia of infancy (PHHI), an autosomal recessive disorder of glucose metabolism which is linked to chromosome 11p15.1 and characterized by unregulated secretion of insulin and profound hypoglycemia. Because the Kir6.2 locus is within 5 kilobases (kb) of the SUR gene on chromosome 11p15.1 and it is a necessary member of the beta-cell KATP channel, we considered Kir6.2 as a candidate gene for PHHL we identified a homozygous point mutation in Kir6.2 in the genomic DNA of a child, severely affected with PHHI, from a consanguineous family. This mutation is predicted to disrupt the conserved alpha-helical second transmembrane (M2) domain of the inward rectifier by substitution of a proline for a leucine residue (L147P). Mutation of Kir6.2, like SUR, appears to lead to the PHHI phenotype suggesting that Kir6.2 is necessary, although not sufficient, for normal regulation of insulin release.