Objective: We attempted to observe the alterations in QTd and QTcd in chronic renal failure (CRF) patients before and after hemodialysis (HD) to determine the relevant determinants of QTc duration in HD. Methods: The HD was carried out 2 or 3 times/week in a standard setting for 4–4.5 h. No drug therapy was applied during HD, except for isotonic NaCl infusions and sodium heparin. Maintenance drug therapy, including digitalis, antihypertensive, anti-anginal, and β-blocking agents, was not changed. In the study, we investigated the alterations in QTd and QTcd in 68 CRF patients before and after HD with 12-lead ECG. Plasma Na<sup>+</sup>, K<sup>+</sup>, ionized Ca, creatinine, urea nitrogen, and hemoglobin were also controlled before and after HD. Results: In our study QTd and QTcd significantly increased at the end of HD (p < 0.01). Plasma Na<sup>+</sup> and K<sup>+</sup> decreased, and ionized Ca increased after HD (p < 0.05, 0.01). Plasma Na<sup>+</sup>, K<sup>+</sup>, ionized Ca levels, ultrafiltration volume and myocardial ischemia appear to be the main determinants of QTc duration in HD, not hypertension, gender, patient age, or duration of chronic HD. Conclusion: Changes in plasma Na<sup>+</sup>, K<sup>+</sup> and ionized Ca, the ultrafiltration volume and presence of ischemic heart disease in HD have significant effects on QTcd. ECG data demonstrate that the risk of arrhythmia could be higher with decreased plasma Na<sup>+</sup> and K<sup>+</sup>, increased ionized Ca, the presence of ischemic heart disease and an increased ultrafiltration rate during HD. These results might provide some valuable references for proper HD programs.