We now report a patient with congenital hyperinsulinism with hemihypertrophy resulting from novel paternally inherited, recessively acting KCNJ11 mutation(s), which have been unmasked by paternal uniparental isodisomy that extends from 11p15.5 to 11p15.1 within the pancreatic tissue. Here, KCNJ11 is linked to hyperinsulinism.