BACKGROUND: We analyzed electromechanical mismatch (EMM) and its relationship to ventricular repolarization in patients with nonischemic dilated cardiomyopathy (DCM). METHODS AND RESULTS: In 39 DCM patients with left ventricular ejection fraction (LVEF) <40% and New York Heart Association functional class >/=III, electroanatomic mapping was used to quantify areas of EMM. High-resolution electrocardiograph was used to measure heart rate variability (HRV) and QT variability index (QTVI). EMM was present in 22 patients (56%, group 1), whereas 17 patients presented no mismatched segments (44%, group 2). The groups did not differ in age (56 +/- 10 years in group 1 vs 57 +/- 7 years in group 2; P = .82), sex (male: 82% vs 94%; P = .40), LVEF (27 +/- 8% vs 25 +/- 6%; P = .18), or N-terminal pro-B-type natriuretic peptide (2,350 pg/mL vs 2,831 pg/mL; P = .32). Although heart rate and HRV were similar in both groups (rate: 80 +/- 20 beats/min in group 1 vs 74 +/- 19 beats/min in group 2 [P = .47]; standard deviation of normal-to normal RR intervals: 106 +/- 79 vs 88 +/- 115 [P = .61]), we found significantly higher QTVI values in patients from group 1 (-1.15 +/- 0.46 vs -1.62 +/- 0.51 in group 2; P = .005). In patients with implantable cardioverter-defibrillators, ventricular arrhythmias recorded </=1 year before enrollment were more frequent in group 1 than in group 2 (58% vs 13%; P = .02). CONCLUSIONS: EMM is present in a majority of patients with DCM and is associated with ventricular repolarization instability.