Q-T prolongation is well-known to be related to ventricular arrhythmias in a number of clinical circumstances. The purpose of this study was to determine whether heart rate corrected Q-T interval (QTc) may indicate susceptibility to ventricular arrhythmias during stress testing. QTc was determined at rest, during submaximal and maximal bicycle work load in normals and patients with remote myocardial infarction, documented coronary artery disease (CAD) and in a group of patients with different cardiac diseases and premature ventricular beats during exercise testing (PVC). Ventricular arrhythmias were graded according to Lown. Q-Tc interval significantly increased in all groups of those patients having complex ventricular arrhythmias (Lown greater than or equal to III). In patients with ischemic S-T segment depression during stress testing, QTc was only lengthened if complex ventricular arrhythmias were present. Q-Tc lengthening in stress testing is assumed to be associated with occurrence of ventricular premature beats rather than ischemia. There is evidence that abnormal repolarization due to imbalance of the autonomic nervous system induces ventricular arrhythmias. Monitoring of Q-Tc interval during stress testing is warranted and may be useful to predict future cardiac events such as sudden cardiac death.