The cardiopulmonary bypass techniques of peripheral cannulations and deep hypothermia provide safe and controlled re-entry into the mediastinum, when the thoracic organs are contiguous with the sternum. In such cases, in order to prevent ventricular distention during cooling, left ventricular venting is very important but can be difficult. We made a small (3 cm) left-sided thoracotomy incision and inserted a left ventricular apical venting tube while cooling a patient with a large pseudoaneurysm of the ascending aorta, which was diagnosed 12 years after aortic valve replacement. We found that this technique was easy, safe, and useful to prevent ventricular distention during cooling.