A 58‐year‐old man with a history of hypertension, dyslipidemia, and an obtuse marginal branch coronary stent developed the sudden onset of chest pain. A contrast computed tomography demonstrated a penetrating ulcer of nondilating ascending aorta and a small pericardial effusion. Coronary angiography showed three‐vessel disease. At emergency surgery, there was blood in the pericardial sac and the whole of the ascending aorta had hematoma with no evidence of tamponade. We performed a hemiarch replacement under circulatory arrest and concomitant coronary artery bypass grafting. The left side of the proximal arch had ruptured just beyond the pericardial reflection.