This 10-month-old girl presented with exertional dyspnea and syncopal episodes. Auscultation demonstrated a systolic ejection murmur. The child's father, paternal uncle, and paternal aunt had been diagnosed with hypertrophic cardiomyopathy. A, B. Radiographs show cardiomegaly and a normal pulmonary vascular pattern. C, D. Echocardiography demonstrates severe myocardial thickening of the left ventricle.