QT-interval prolongation is a recognized risk indicator for ventricular arrhythmias and sudden cardiac death. The effect of beta-blockers on the change of the QT-interval relative to the change in heart rate was studied in 269 male patients who underwent computer-interpreted exercise tests. None of the patients studied used anti-arrhythmic drugs, diuretics or digoxin. In 141 men on beta-blockers the relation between Q-peakT interval and heart rate could be described as follows: Q-peakT = -1.48 X heart rate + 415. In 128 patients not on beta-blockers this relation was: Q-peakT = -1.14 X heart rate + 379. The difference of the slopes is 0.34 (0.22 - 0.46, 95% confidence interval). This difference was even more pronounced in a subgroup of coronary patients: 0.44 (0.30 - 0.57, 95% confidence interval). These results indicate that in men using beta-blockers the QT-interval shortens faster with increasing heart rate than in men not using them. At high heart rates patients on beta-blockers have a shorter QT-interval than those not using them. These observations could explain the beneficial effect of beta-blockers on exercise-induced ventricular arrhythmias and sudden death in coronary patients.