Background QT interval dispersion, measured as interlead variability of QT, is a marker of dispersion of ventricular repolarization and, hence, of cardiac electrical instability. We tested the hypothesis that dispersion of ventricular repolarization may be differently affected by interventions destined to provide complete or incomplete protection against malignant arrhythmias in patients with long QT syndrome (LQTS). Twenty-eight patients affected by the Romano Ward form of LQTS entered the study and were divided into three groups: LQTS patients before institution of therapy, patients who did respond to beta -blocker therapy, and patients who continued to have syncope and cardiac arrest despite beta -blockade and who underwent left cardiac sympathetic denervation. A group of 15 healthy volunteers served as control subjects.Methods and Results Dispersion of QT and QT sub c were calculated using two indexes: the difference between the longest and the shortest value measured in each of the 12 ECG leads (QTmax-QTmin, QTcmax-QTcmin) and the relative dispersion of QT and QTc(standard deviation of QT/QT average x 100, standard deviation of QTc/QTcaverage x 100). Both indexes of dispersion of repolarization were higher in the LQTS patients than in control subjects; also, patients not responding to beta -blockers had a significantly higher dispersion of repolarization than responders. A cutoff value of 100 milliseconds for QTmax-QTminhad an 80% sensitivity and 82% specificity in discriminating between responders and nonresponders. A cutoff value of 6 for QT relative dispersion yielded similar results. The LQTS patients who did not respond to beta -blockade underwent left cardiac sympathetic denervation and thereafter remained asymptomatic (mean follow-up, 5+-4 years). In this group, dispersion of repolarization was significantly reduced by the surgical denervation to values similar to that of the responders to beta -blockade.Conclusions These data indicate that QT dispersion is a useful clinical tool to predict efficacy of antiadrenergic therapy in LQTS patients. (Circulation. 1994;89:1681-1689.)