Cardiac resynchronization therapy (CRT) is theoretically expected to affect repolarization as well as depolarization. We studied the effects of CRT on corrected QT (QTc) dispersion in association with symptomatic improvement. QTc dispersion was analyzed in 26 consecutive patients (67 +/- 6 years old, 18 men and 8 women) who underwent CRT. CRT responders and nonresponders were defined as patients showing and not showing > or = 1 class New York Heart Association symptomatic improvement 3 months after CRT, respectively. QTc interval, QRS width, and QTc dispersion were measured automatically from digital data using an analyzing system. There were 18 CRT responders and 8 nonresponders among the patients. CRT responders showed significantly larger QTc dispersion than CRT nonresponders before CRT (102 +/- 26 vs 40 +/- 12 ms, P < 0.01). A significant decrease in QTc dispersion by CRT was observed in responders (102 +/- 26 to 52 +/- 15 ms, P < 0.01). In contrast, QTc dispersion was not decreased by CRT in nonresponders (40 +/- 12 to 39 +/- 11 ms, not significant). The difference observed before CRT was thus abolished after CRT (52 +/- 15 vs 39 +/- 11 ms, not significant). Baseline values and changes in QRS width or QTc, as well as asynchrony of wall motion determined by tissue Doppler imaging, were not different between CRT responders and nonresponders before CRT. The present study with a small number of patients shows the potential utility of QTc dispersion for distinguishing CRT responders from CRT nonresponders before CRT, and warrants further study with a greater number of patients.