A rare case of glycogen storage disease type III with unusually absent ketone body production during hypoglycemia is presented. A 10-month-old boy presented with asymptomatic hepatomegaly. GOT/GPT 2555/1160 IU/L, CK 302 IU/L, triglycerides 1223 mg/dL, cholesterol 702 mg/dL and uric acid 7.9 mg/ dL. After a 9-hour fast, glucose was 27 mg/dL and adequate lipolysis without ketogenesis was observed (total/free carnitine 34.5/20 μmol/L, free fatty acids 1620 μmol/L and β-hydroxybutyrate 172 μmol/L). Result of MCT (medium-chain triglycerides) load test: basal β-hydroxybutyrate 29 μmol/L rose to 5748 μmol/L. Treatment with a fat-restricted diet supplemented with formula containing MCT was initiated and the patient presented a satisfactory initial evolution. Three months later, CK were 3000 IU/L. Muscle biopsy was diagnostic of glycogenosis. Enzymatic activity in skin fibroblasts was 0% for amylo-1,6-glucosidase. The diagnosis of glycogenosis type III was established. Echocardiography performed at that time showed non-obstructive ventricular hypertrophy. Until now hypoketosis during hypoglycemia has only been described in glycogenosis type I. © Freund Publishing House Ltd.