Congenital aneurysms of the muscular interventricular septum without ventricularseptal defect are extremely rare. We describe a five-month-old girl patient with congenital aneurysm, pathological ST changes and minimal right ventricular outflow tract obstruction. The electrocardiogram showed Wolf-Parkinson-White syndrome, pathological ST changes and combined ventricular hypertrophy criteria. The transthoracic echocardiography and cardiac catheterization demonstrated a septal aneurysm which was bulging into the right ventricle and systolic pressure gradients between the main pulmonary artery and right ventricle outflow tract with normal coronary arteries. The patient has shown no symptoms, but some rhythm disturbances may be expected because of her electrocardiographic anomalies. Therefore, these patients should be followed carefully because of possible complications.