1. Three cases of glycogen storage disease occurring in the early neonatal period are presented. Two of these were in siblings, 1 of whom died at the age of 1 day. 2. Tachypnea and enlargement of the liver were the prominent manifestations of the illness. The similarity between this syndrome and other syndromes points up the need for instituting suitable clinical and pathologic diagnostic procedures in the early neonatal period. 3. Determinations of glycogen and glucose-6-phosphatase in the liver in 2 of these patients are presented. 4. Quantitatively standardized glucagon tolerance curves are presented in 2 of these patients and compared with those obtained in normal infants. The character of the response suggests its value as a diagnostic test in this condition. 5. Electroencephalographic tracings during periods with marked hypoglycemia and during periods with normal values for blood sugar were found to be essentially normal. It is suggested that some unusual mechanism for the maintenance of cerebral metabolism during hypoglycemia may be present.