Background The measurement of QT dispersion in the surface electrocardiogram is a noninvasive method used for assessing inhomogeneity of myocardial repolarization. Elevated QT dispersion is found in myocardial disease and is associated with an increased incidence of arrhythmic events. QT dispersion is also increased in myocardial hypertrophy secondary to systemic hypertension. However, the relation between left ventricular (LV) enlargement in endurance trained subjects and QT dispersion is unknown. Methods and Results In this study, LV mass (2-dimensional echocardiography) and QT dispersion (12-lead resting electrocardiogram) were assessed in 26 normotensive endurance trained subjects and 26 matched, less trained control subjects. Endurance trained subjects had a significantly greater LV mass (216 ± 39 g vs 155 ± 30 g, P < .001) but lower heart rate–corrected QTc dispersion (42 ± 13 ms vs 51 ± 15 ms, P = .012) than less trained control subjects. When all individuals were included, LV mass was inversely correlated with QT dispersion (r = –0.38; P = .002) and heart rate–corrected QTc dispersion (r = –0.53, P < .0001). Conclusions These data show that myocardial hypertrophy induced by exercise training is not associated with increased QT dispersion as observed in systemic hypertension. The reduced QT dispersion reflects homogeneous myocardial repolarization and may help to explain the reduced mortality rate in regularly exercising subjects. If confirmed in further studies, the measurement of QT dispersion could provide a simple and inexpensive screening method for differentiating between physiologic and pathologic myocardial hypertrophy. (Am Heart J 1999;138:309-12.)