Congenital hyperinsulinism is an important cause of persistent hypoglycemia in neonates. We present a term large-for-gestation neonate with congenital hyperinsulinism, who was found to have a novel sporadic missense mutation in the ABCC8 gene. The clinical phenotype of our case is described along with results of genetic testing. Our patient had an early onset of persistent hypoglycemia, which responded to diazoxide and octreotide. The echocardiogram revealed diffuse hypertrophy of the ventricular walls and septum, which regressed spontaneously by 8 months of age. The specific ABCC8 missense mutation has not been previously reported in association with congenital hyperinsulinism. Our case highlights the need for genetic evaluation in this condition. The unraveling of new mutations with unique phenotypic features may have diagnostic and prognostic utility.