A case of interventricular septal rhabdomyoma in a one day-old newborn is presented. The newborn was moderately cyanosed. A 3/6 systolic murmur in basal area was present. An electrocardiogram showed the features of Wolff-Parkinson-White syndrome. A chest x-ray showed distinct cardiomegaly. M-Mode (MME) and Two-dimensional echocardiography (2DE) identified an interventricular septal mass within left ventricle and suggested the presence of left ventricular outflow obstruction. We emphasize the usefulness of MME and 2DE in accurately diagnosing intramural ventricular tumors in newborns.