Defective activity of carnitine palmitoyltransferase I was demonstrated in fibroblasts derived from a patient with hypoketotic hypoglycemia. The level of activity observed was approximately 10% of the control mean. Oxidation of palmitate by intact fibroblasts was reduced to 5% of control values. The patient presented at age 14 months with seizures and was found to have marked hypoglycemia and no ketones in the urine. In response to fasting, she developed hypoglycemia, but the curves for acetoacetate and 3-hydroxybutyrate were flat. Administration of medium-chain triglycerides relieved the hypoglycemia and generated a brisk ketogenesis.