This is a case report of an uncommon form of subaortic stenosis caused by an abnormal accessory mitral leaflet simulating a left ventricular outflow tract tumor. A 5-year-old boy with a systolic heart murmur underwent cardiac catheterization which demonstrated a subaortic stenosis with a 90 mmHg peak systolic pressure gradient. Angiography showed a tumor-like structure in the left ventricular outflow tract. At operation this mass was found to be an abnormal bulky accessory mitral leaflet. Resection of this deformed leaflet was carried out successfully.