Increased QT interval and QT dispersion have been associated with higher mortality in population-based studies and in patients with myocardial infarction. However, the prognostic significance of these measurements in patients with left ventricular (LV) systolic dysfunction is not clear. Rate corrected QT interval (QTc) and QT dispersion (QTd) were measured by means of an automated method from digitized echocardiograms in 2265 patients with an LV ejection fraction < or =40% and were related to survival. Increased QTc was strongly related to mortality in the whole group and in subsets on the basis of age and the level of LV systolic dysfunction. There was a graded increase in mortality rate with an increase in QTc. The effect of QTc on mortality was incremental to the effects of age and ejection fraction. QT interval was measurable in > or =6 leads in 1193 patients in whom QTd was computed. QTd higher than the mean value of 35 ms was associated with an increase in all cause mortality (P =.04). Its mortality impact was most pronounced in the older patients, patients with more severe LV dysfunction, and patients with increased QTc. Both QTc prolongation and increased QTd are associated with higher mortality rate in patients with moderate and severe LV dysfunction.