Increased QT interval dispersion is associated with ventricular arrhythmias. The aim of this study was to examine if in postmyocardial infarction patients with impaired left ventricular function, increased QT dispersion is associated with ventricular tachycardia (VT) and ventricular fibrillation (VF). Measures of QT dispersion, calculated as maximum-minimum (D) and standard deviation (SD) of QTend, QTapex, JTend, JTapex, and Tend intervals in the 12-lead electrocardiogram, were compared in patients who late after myocardial infarction experienced sustained VT (VT group) only, VF (VF group) only, or had no ventricular arrhythmias (controls). The 25 patients in each group were individually matched for age, gender, number of diseased coronary vessels, location of the previous myocardial infarction, and left ventricular ejection fraction. Dispersion measures of QTend, QTapex, and JTapex intervals separated VT group from controls, but none of the measures separated the VF group from controls. QTendD was 49+/-18 ms in controls, 57+/-18 ms in the VF group (controls vs VF group, p = NS), and 65+/-29 ms in the VT group (controls vs VT group, p <0.05). VT group had increased QTapexSD, JTapexSD, and JTapexD compared with the VF group. The cycle length of induced sustained monomorphic VT, present in 19 VT and 19 VF patients, correlated with several dispersion indexes in the VT group, but not with those in the VF group. Thus, in postmyocardial infarction patients with a severely damaged left ventricle, increased QT dispersion is associated with susceptibility to sustained VT, but not to VF.