We describe a patient with left ventricular outflow tract obstruction after mitral valve replacement preserving the anterior subvalvular apparatus. Postoperative transesophageal echocardiography demonstrated systolic narrowing of the left ventricular outflow tract by a bulging septum and systolic anterior motion of the preserved anterior mitral leaflet. Septal myectomy and transaortic mitral apparatus resection enabled us to relieve the left ventricular outflow tract obstruction. This suggests that septal hypertrophy might be a relative contraindication to the preservation of the anterior mitral subvalvular apparatus in mitral replacement.