We report an unusual case of left-ventricular outflow obstruction caused by accessory mitral valve tissue associated with dextrocardia and ventricular septal defect in a seven-day-old, 2200 grams premature infant, who was referred with a heart murmur. The diagnosis was made by two-dimensional and Doppler echocardiography which demonstrated the accessory tissue as well as a 100 mmHg peak systolic gradient between the left ventricle and the aorta. Ten days after the presentation the infant underwent emergency surgery after respiratory arrest and recurrent episodes of syncope. The accessory mitral valve tissue and its fibrous extension were excised and the ventricular septal defect was closed. We believe that surgical treatment of patients with accessory mitral valve tissue should be performed early because of the possibility of acute deterioration. Combined aortotomy and interatrial approach is very helpful in evaluating the anatomy and the mitral valve function as well as delineating the tissue to be excised.