Pompe disease (glycogen storage disease II, OMIM # 232300), is a hereditary lysosomal disorder. It is characterized by deficiency of acid alpha-glucosidase enzyme (acid maltase, GAA, OMIM *606800, EC 220.127.116.11), secondary to mutations in the GAA gene (HGNC:4065) on chromosome 17q25.2-q25.3. Absent enzyme activity in the infantile form of Pompe disease results in abnormal glycogen deposition in the skeletal, cardiac, and smooth muscles, leading to hypertrophic cardiomyopathy, feeding abnormalities, hypotonia, weakness, respiratory insufficiency, and ultimately death. Prenatal diagnosis is accomplished by enzyme assay, mutation analysis or electron microscopy of amniotic fluid cells or chorionic villus sample. However, these techniques may not always be available, and can result in perinatal morbidity and fetal loss. Early diagnosis of Pompe disease results in early institution of enzyme replacement therapy (ERT), which minimizes morbidity and prolongs survival. We report the case of a 35-week part-of-twin neonate, whose older sibling died earlier because of infantile Pompe disease. At 32 weeks of gestation, fetal echocardiography showed hypertrophic cardiomyopathy in twin 1, which persisted until birth at 35 weeks of gestation. Diagnosis was confirmed after birth by enzyme assay, and mutation analysis showing homozygosity for the sequence change 1327-2A>G (GAA intr 8). Administration of ERT at 18 h of age, resulted in normalization of cardiac abnormalities within 21 weeks of therapy, and normal neurodevelopmental assessment at 46 weeks, using Griffiths Mental Development Scales. To our knowledge, this is the youngest patient reported to receive ERT for Pompe disease, and the first report of prenatal diagnosis of Pompe disease by fetal echocardiography.