Prader-Willi Syndrome (PWS) is a complex genetic disorder associated with varied clinical findings, neurocognitive delay, and endocrine abnormalities. Clinically, individuals with PWS progress along a path marked by different nutritional stages. In infancy, children with PWS have hypotonia, poor feeding, excessive daytime sleepiness, and difficulty gaining weight. Interestingly, patients with recurrent neonatal hypoglycemia present with similar symptoms. The overlap in clinical presentations of these two conditions raises the possibility that hypoglycemia may explain some of the clinical findings associated with PWS. Studies in PWS mouse model further support the presence of hypoglycemia in PWS (Stefan et al 2011). These mice had decreased glucagon and insulin levels resulting in a relative hyperinsulinemia. Because there have been no studies of hypoglycemia in humans with PWS, we reviewed the medical charts of 95 infants with PWS. We found documented neonatal hypoglycemia in a significant number of patients. In our proposed observational study, we aim to further characterize hypoglycemia in PWS phase 1a. Our study will involve review of past medical records, admission for controlled fast, and biochemical testing. We believe further study of glucose metabolism will lead to key insights into pathophysiology of the disease, set the stage for trials of therapy, and lead to improved neurodevelopmental outcomes for children with PWS.