Background: We sought to investigate (1) the relationship between QRS duration on electrocardiogram and right ventricular (RV) volumes determined by cardiac magnetic resonance imaging (CMR) and (2) the usefulness of change in QRS duration in predicting a significant RV reverse remodeling in patients with corrective surgery for severe functional tricuspid regurgitation (TR). Methods and Results: We enrolled 30 consecutive patients (27 women, aged 57.8짹9.6 years) with severe TR who had undergone prior left-sided valve surgery. CMR and 12-lead standard surface ECG were performed before and long after corrective TR surgery. Computer-assisted analysis program was employed for QRS duration calculation. At a median follow-up of 27.5 months, corrective TR surgery cut QRS duration by 14.6%, from 110.4짹14.6msec before to 96.9짹11.9msec after TR surgery (P<0.001), while CMR showed a decrease in RV end-diastolic volume index (RV-EDVI) from 179.559.7 to 119.130.4 mL/m2 (P<0.001). QRS duration displayed a significant correlation with RV-EDVI and RV end-systolic volume index (r=0.65, P<0.001 and r=0.53, P<0.001, respectively). A percent change in QRS duration was significantly correlated with a percent change in RV-EDVI (r=0.40, P=0.03). When a significant RV reverse remodeling was defined as a reduction in RV-EDVI ��� 20% after corrective TR surgery, sensitivity and specificity for a significant RV reverse remodeling were 75% and 78%, respectively, with a cut-off value of 9% in terms of a percent reduction in QRS duration (P=0.01, area under the curve = 0.81) Conclusion: QRS duration on 12-lead surface ECG can be a proxy for the extent of RV volume overload in patients with corrective TR surgery. Postoperative change in QRS duration can be used as a useful, simple index in screening the occurrence of RV reverse remodeling in patients undergoing corrective TR surgery.