The ketogenic diet is a rational treatment for pyruvate dehydrogenase complex deficiency (McKusick 312170) and GLUT1 deficiency syndrome (McKusick 138140). An increasing number of patients are diagnosed in early infancy, but few data are available on the introduction of a ketogenic diet in this age group. GLUT1 deficiency syndrome was suspected in four infants presenting with seizures and unexplained hypoglycorrhachia. A ketogenic diet was introduced at 6-28 weeks of age. Ketosis was initiated by fasting, monitored by bedside blood glucose and 3-hydroxybutyrate determinations, and was maintained successfully using supplemented carbohydrate-free infant formula and emulgated triglycerides. All patients developed ketosis within 24 h. 3-Hydroxybutyrate concentrations available at the bedside correlated inversely with the base excess. At glucose levels < or = 40 mg/dl patients remained asymptomatic in the presence of ketones. The ketogenic formula was tolerated well, parental compliance was good, and all patients remained seizure-free on the diet. GLUT1 deficiency was confirmed in two patients; the diet was discontinued in the other two patients. In one infant, failure to thrive on medium-chain triglycerides was effectively reversed using long-chain triglycerides. Urine dipstick analyses failed to detect ketosis in another infant. Adverse effects of the diet were limited to renal stones in one patient. The ketogenic diet can be introduced and maintained successfully in young infants using long-chain fat emulsion. Monitoring 3-hydroxybutyrate at the bedside was useful for metabolic control and superior to urine dipstick analysis. Seizure control was effective and adverse effects were limited, but evaluation of the long-term effects of the ketogenic diet in this age group must await ongoing studies.