Severe QT interval prolongation >500ms occurs in one quarter of cardiac surgical patients in the perioperative period while moderate prolongation occurs in most of them. Prolonged QT interval may be associated with torsades de pointes and lead to sudden cardiac death. Because of the high incidence of prolonged QT in cardiac surgery patients and its perioperative adverse outcomes, it is vital to identify it early and take necessary precautions. We report and discuss the catastrophic events and management of two patients with long QT syndrome complicating mitral valve replacement.