Aims To examine whether the presence of pre-pacing functional mitral regurgitation (MR) and its improvement would affect the extent of left ventricular (LV) reverse remodelling after cardiac resynchronization therapy (CRT). Methods and results Echocardiographic assessment was performed in 83 patients before and 3 months after CRT. Total MR volume and the early-and late-systolic MR flow rate were assessed. At 3 months, there was reduction in total MR volume (38 ± 20 vs. 33 ± 21 mL) with decrease in both early-(71 ± 52 vs. 60 ± 51 mL/s) and late-systolic (49 ± 46 vs. 42 ± 46 mL/s) MR flow rate (all P < 0.05). Receiver-operating characteristic curve found that an 11 decrease in total MR volume was associated with LV reverse remodelling [defined by the reduction in LV end-systolic volume (LVESV) of ≥15] [sensitivity, 90; specificity, 80; area under the curve (AUC), 0.85; P < 0.001]. The improvement in early-and late-systolic MR was also associated with LV reverse remodelling, in which improvement in early-systolic MR had higher sensitivity, specificity, and AUC than late-systolic MR. The extent of reverse remodelling with gain in LV ejection fraction and forward stroke volume was greatest in patients with improvement in total MR, intermediate in those with mild or no MR at baseline, and the least in those without improvement in total MR (LVESV,-29.8 ± 12.0 vs.-18.6 ± 16.6 vs.-5.5 ± 8.6; ejection fraction, 11.8 ± 6.2 vs. 7.0 ± 6.8 vs. 3.0 ± 5.0; forward stroke volume, 43.1 ± 37.9 vs. 21.1 ± 26.1 vs. 6.8 ± 34.6; all P < 0.05). Conclusion Improvement of functional MR contributes to LV reverse remodelling after CRT, whereas reduction of early-systolic MR is more powerful than late-systolic MR. © 2010 The Author.