Pompe disease ( PD), is an autosomal recessive disorder caused by a deficiency of acid alpha glucosidase resulting from the pathogenic variants in GAA. According to the clinical characteristics and age of disease onset, PD is categorized as infantile and late-onset forms. Infantile onset Pompe disease (IOPD) is characterized by hypotonia, respiratory difficulty and hypertrophic cardiomyopathy (HCM). This study described the clinical manifestations and genetic analysis of 5 patients (2 males, 3 females) with IOPD in a single center in Korea. Their median age presenting initial symptom was 7 months (range: 2 months-1 year). Common cheif complaints were respiratory distress and weakness of lower extremites. All patients showed vacuolation of many fibers on skeletal muscle biopsy and decreased acid alpha glucosidase activity in peripheral blood leukocytes or muscle fiber. Their median age at diagnosis was 3.8 years (range: 2 months-15 years). Electrocardiography at diagnosis revealed HCM in three patients with IOPD. Two patients received respiratory support with home ventilators. All patients showed hepatomegaly and elevated liver transaminase. Splenomegaly, scoliosis, and valgus deformity were noted in two, three, and three patients, respectively. All patients received enzyme replacement therapy (ERT) with alglucosidase alfa. The median age at starting ERT was 5.12 years (range: 1 -16 years). Three patients with HCM showed the reduction of left ventricular muscle mass and increased ejection fraction on echocardiogram after ERT. The size of liver and level of liver transaminase were also decreased for all patients. However, two patients who had gotton ventilator support before ERT did not showed improvement of respiratory function after ERT. Direct sequencings of GAA revealed 10 different mutant alleles from 5 patients with IOPD including three nonsense, five missense mutations, and two deletions. There was no hot spot. In conclusion, ERT reduced heart hypertrophic mass and liver size, however, did not improve respiratory function of patients with IOPD. Early diagnosis and treatment are important for patients with IOPD.