Idiopathic ketotic hypoglycemia is the most frequent cause of hypoglycemia in children between 1 and 5 years of age. The symptoms and signs of hypoglycemia are often overlooked because they mimic signs of other common diseases like psychiatric disorders, migraine, gastro-enterological dysfunction, or visual disturbances. Glycemia and ketone bodies in the urine should be systematically investigated in such cases. Because hypoglycemia is a life-threatening event and can lead to severe neurological sequelae, intravenous administration of glucose is mandatory. These children respond promptly to glucose. Infants with normal growth and psychomotor development, normal physical examination who present with a first episode of symptomatic fasting hypoglycemia and elevated ketonuria, and who improve quickly after intravenous glucose administration, do not need a comprehensive metabolic and endocrine workup. Recurrence of hypoglycemic attacks can be prevented by supplying frequent snacks containing complex carbohydrates, so called "slow sugars", particularly at bed-time. Other causes of ketotic hypoglycemia are briefly presented.