With its unique en face view, real time three-dimensional transesophageal echocardiography has been reported to be more precise than conventional two-dimensional studies in evaluating mitral regurgitation etiology, and can locate diseased segments correctly. We present a case with severe mitral regurgitation due to anterior mitral leaflet perforation. Intraoperative real time three-dimensional transesophageal echocardiography demonstrated its value in diagnosis and surgical planning for this perforation, which had not been identified preoperatively. This technique should be applied more widely for dedicated mitral valve assessment in clinical practice.