We experienced a case of glycogen storage disease(type Iia) in a 11 months old girl who was admitted to Pediatric service of Kangnam St. Mary's hospital for work-up of flaccidity and developmental delay. The baby was relatively well until 3 months ol age when she began to have poor sucking and swallowing, and also her crying was weak. The patient has been markedly behind in all her developmental milestones and revealed hypotonia uhich was apparent on ventral and vertical suspensions. The chest X-ray film showed cardiomegaly, and echocardiography was done twice to get a diagnosis of hypertrophic cardiomyopathy. ECG showed biventricular hypertrophy. The brain CT showed no abnormality. Needle EMG showed fibrillation and positive sharp waves typical of a myopathy. Total CPK was 349 IU/L with an increase in fraction of MM band. Light and electronmicroscopic findings of muscle biopsy were compatible with Pompe's disease of infantile type.