We report a successful case of mitral valve replacement and coronary artery bypass grafting under mild hypothermia and systemic hyperkalemia in a patient with severely atheromatous ascending aorta on which placing a clamp seemed contraindicated. A 78-year-old man was referred to our hospital with the diagnosis of heart failure associated with severe mitral regurgitation and coronary artery disease. Echocardiography showed severe mitral regurgitation due to A3, P3 and posterior commissure (PC) prolapse and coronary angiography showed three vessel disease. Computed tomography( CT) revealed a severely atheromatous ascending aorta. Surgery was performed under cardiac arrest using systemic hyperkalemia and superior transseptal approach. Although cardiopulmonary bypass (CPB) time was a little prolonged in order to wash out potassium with dilutional ultrafiltration, the patient was uneventfully separated from CPB. The patient had no neurological complications and was discharged from the hospital 15 days after surgery. Mitral valve replacement under cardiac arrest using systemic hyperkalemia without cross clamping the aorta is useful to avoid neurological complications.