Beckwith-Wiedemann syndrome (BWS) is an overgrowth syndrome associated with macrosomia, macroglossia, abdominal wall defects, hypoglycemia in the neonatal period and embryonal cancers of infancy and early childhood. The frequency of hypoglycemia in this population is between 30% and 50%. The majority of infants with hypoglycemia will be asymptomatic and have resolution of the hypoglycemia within the first 3 days of life. Less than 5% will have hypoglycemia beyond the neonatal period requiring either continuous feeding or a partial pancreatectomy. The cause of hypoglycemia is unclear, but direct and indirect evidence supports a hyperinsulinemia as the major factor. Recent identification of the majority of genes associated with BWS in the 11p15 region and the genotype of persistent hyperinsulinemia hypoglycemia of childhood also in the 11p15 region may provide a molecular basis for hypoglycemia in BWS, particularly for the occasional patients with hypoglycemia requiring a partial pancreatectomy. Detailed genotype phenotype evaluations are needed and should provide an insight as to why patients with BWS have hypoglycemia. Copyright (C) 2000 by W.B. Saunders Company.