Accessory mitral valve tissue is a rare cause of subaortic obstruction. The reported case correspond to a two days old patient diagnosed by 2D-Echocardiography. Serial doppler flow analysis showed progressive left ventricular outflow obstruction. A cardiac catheterization at the age of 9 months confirmed the obstruction without defining the cause. Five months later, transesophageal echocardiography clearly defined the intraventricular connections of the accessory mitral tissue. At the age of 18 months the patient suffered from cardiac failure and underwent surgery for removing the accessory tissue, assisted by intraoperative transesophageal echocardiography pre and post by-pass. Follow-up echocardiography at 25 months showed no left ventricle outflow obstruction and the patient is now asymptomatic.