Background: Beckwith-Wiedemann syndrome (BWS) is a congenital overgrowth syndrome that is clinically and genetically heterogeneous. Hyperinsulinemic hypoglycemia occurs in about 50% of children with BWS and, in the majority of infants, it resolves spontaneously. However, in a small group of patients the hypoglycemia can be persistent and may require pancreatectomy. The mechanism of persistent hyperinsulinemic hypoglycemia in this group of patients is unclear. Patients and Methods: Using patch-clamp techniques on pancreatic tissue obtained at the time of surgery, we investigated the electrophysiological properties of ATP-sensitive K+ (KATP) channels in pancreatic β-cells in a patient with BWS and severe medically-unresponsive hyperinsulinemic hypoglycemia. Results: Persistent hyperinsulinism was found to be caused by abnormalities in KATP channels of the pancreatic β-cell. Immunofluorescence studies using a SUR1 antibody revealed perinuclear pattern of staining in the BWS cells, suggesting a trafficking defect of the SUR1 protein. No mutations were found in the genes ABCC8 and KCNJ11 encoding for the two subunits, SUR1 and KIR6.2, respectively, of the KATP channel. Genetic analysis of this patients BWS showed evidence of mosaic paternal isodisomy. Conclusions: In this novel case of BWS with mosaic paternal uniparental disomy for 11p15, persistent hyperinsulinism was due to abnormalities in KATP channels of the pancreatic β-cell. The mechanism/s by which mosaic paternal uniparental disomy for 11p15 causes a trafficking defect in the SUR1 protein of the KATP channel remains to be elucidated. Copyright © 2005 by The Endocrine Society.