Increased dispersion of QT intervals is known to predispose to ventricular arrhythmias and sudden cardiac death. To assess the effect of hemodialysis (HD) on QT and corrected QT (QTc) intervals and their dispersions in chronic hemodialyzed patients we studied 85 patients (male/female = 48/37; mean age 44 +/- 17 year) on chronic hemodialysis. Simultaneous 12-lead ECG was recorded before and after HD in a standard setting. The QT intervals for each lead were measured manually by one observer using calipers. Each QT interval was corrected for heart rate: QTc= QT/ mean square root of R-R (in milliseconds [ms]). ECG parameters, body weight, blood pressure, heart rate, electrolytes (Na+, K+, Ca++, phosphate), urea, and creatinine were measured before and after HD. The mean of pre and post dialysis cycle intervals was 828 +/- 132 ms and 798 +/- 122 ms respectively; the difference was not significant. The mean of QTmax intervals changed significantly from 446 +/- 47 to 465 +/- 72 ms (P < 0.05). The mean of corrected QTcmax intervals increased significantly from 472 +/- 38 to 492 +/- 58 ms (P < 0.05). The mean of QT dispersions and the corrected QT interval dispersions changed from 60 +/- 29 to 76 +/- 32 ms (P < 0.05) from 72 +/- 46 to 98 +/- 56 ms (P < 0.05), respectively. During HD, the serum potassium and phosphate levels decreased whereas the calcium levels increased. We conclude that QT and QTc interval and dispersion increase in HD patients.