Successful surgical treatment of subaortic stenosis caused by an accessory mitral valve. A 22-month-old boy with subaortic stenosis was found to have relatively mature mitral valve tissue beneath the aortic valve, associated with a hypertrophic and prominent interventricular septum. This tissue caused obstruction of the left ventricular outflow tract and resulted in a pressure gradient of 70 mmHg between the aorta and the left ventricle. Surgical treatment was successfully performed to excise the tissue and part of the hypertrophic ventricular septum. Results of microscopic examination of the resected specimen are shown and discussed. 