The presence of accessory mitral valve tissue in the left ventricular outflow tract (LVOT) is a rare cause of obstruction. Previous cases reported in the literature are associated either with other complex congenital abnormalities or with other forms of LVOT obstruction. This report presents the first case of a duplication of the anterior leaflet of the mitral valve occurring as an isolated congenital anomaly. It produced mild subaortic obstruction. The report emphasizes the importance of transthoracic two-dimensional echocardiography and Doppler assessment in the recognition of congenital abnormalities of the mitral valve and subvalvular apparatus as a differential diagnosis of subaortic stenosis.