A 61-year-old man was treated medically under a diagnosis of hypertrophic obstructive cardiomyopathy 7 years previously. He was transferred to our hospital because of sudden onset of dyspnea. Chest radiography showed marked pulmonary congestion. Echocardiography revealed hypertrophic obstructive cardiomyopathy, with a left ventricular pressure gradient of 120 mmHg, accompanied by severe mitral regurgitation due to ruptured chordae tendineae. Mitral valve replacement and the Morrow operation were performed. After the operation, no left ventricular outflow pressure gradient was detected by echocardiography and cardiac catheterization. The patient did well and no further cardiac symptoms were detected for 3 years.