We herein report on a male infant who was diagnosed as having a cardiac tumor at the age of four days. This baby was quite well after birth, with the exception of a heat murmur heard by the pediatrician in a routine physical examination. Echocardiograms disclosed a tumor located in the muscular portion of the interventricular septum, almost occupying the whole right ventricular chamber. The patient underwent near total excision of the tumor to relieve the right ventricular outflow obstruction when he was 54 days old. The pathologic report revealed a rhabdomyoma.