Objective: To 1) determine the prevalence of hypoglycemia in childhood in a pediatric emergency department (ED), 2) determine epidemiology of idiopathic ketotic hypoglycemia (IKH), 3) determine diagnostic yield of the workup of hypoglycemia, and 4) review a diagnostic approach to hypoglycemia. Setting: Urban pediatric ED of a tertiary level children's hospital Methods: Retrospective review of all medical records with a primary or secondary diagnosis of hypoglycemia (ICD-9 code 251.2) seen at the ED between 1/92 and 8/95. Results: Thirty-one patients were identified. Mean blood glucose was 34.2 mg/dl. Prevalence of hypoglycemia among population seeking care in our ED was 6.54/100,000 visits. Eighteen patients were diagnosed with IKH for a prevalence of 3.9/100,000. IKH demographics were: mean age 27.7 months; 12 males, 6 females; 8 white, 9 black, and 1 not available. The weights of five patients were < 25th percentile. Fourteen of the 18 IKH patients had hormone studies done insulin [cost $40], growth hormone [$69], cortisol [$54]. All 14 had appropriately suppressed insulin levels (< 5 μU/ml) and high cortisol levels > 22 μg/ml. Thirteen of the 14 had normal or high growth hormone (GH) levels (0.7-6 ng/ml). Four IKH patients had urine drug screens ($280); all were negative. Although no IKH patient was febrile, six had sepsis workups ($380); all were negative. Urine ketones were positive in 15 of the 18 tested (> 3+ in eight patients). Mean anion gap was 20 (range: 16-30). Eight of the 18 IKH patients were discharged from the ED after return to normal status. Conclusions: IKH is the most common cause of hypoglycemia in children beyond the infancy period. In its typical presentation (previously healthy one- to five-year-old, with normal growth and development, who presents with a first episode of symptomatic fasting hypoglycemia and appropriate degree of ketonuria, without hepatomegaly, and with resolution of symptoms on administration of glucose), an extensive and overzealous workup for endocrinopathy or inborn error of metabolism is not necessary.