Septal myectomy and mitral valve surgery are performed for the treatment of hypertrophic obstructive cardiomyopathy (HOCM) ; however, there is a debate as to which is the better option. In this paper, we report on the eventless postoperative course of a patient who underwent septal myectomy and mitral valve replacement simultaneously to treat HOCM accompanied by severe mitral regurgitation. A 75-year-old woman was referred to our department for severe mitral regurgitation accompanied by exertional dyspnea; we observed systolic anterior motion of the anterior mitral leaflet and a left ventricular outflow tract pressure gradient of 130 mmHg that was not decreased by DDD pacing. A 15×6×15-mm rectangular parallele pipe-shaped section of the hypertrophied basal septum was resected via a transaortic approach. In addition, mitral valve replacement with a 25-mm SJM was performed during the same procedure, as lesions were observed in the mitral valve. After surgery, the left ventricular outflow tract pressure gradient decreased by 12.5 mmHg. Necessary and sufficient resection of the hypertrophied basal septum, after careful preoperative and intraoperative evaluation of the extent of myocardial resection and mitral valve lesions is important for improving left ventricular outflow tract pressure gradient in HOCM. Mitral valve replacement should also be performed, if necessary, to treat mitral valve lesions.