BACKGROUND Right ventricular (RV) apical pacing results in abnormal left ventricular (LV) electrical and mechanical activation and is associated with an increased risk of developing heart failure. Chronic RV septal pacing has been shown to be superior to RV apical pacing in newly implanted patients. However, whether RV septal pacing can reverse deleterious effects of RV apical pacing remain unclear. METHODS We evaluated the effects of RV septal pacing on LV performance and functional capacity before and at 18 months after device replacement in 12 patients with previously permanent RV apical pacing and in 12 control patients that continued RV apical pacing. All patients underwent radionuclide ventriculography and 6-minute hallwalk (6-MHW) test before replacement (baseline) and at 18 months afterward to determine changes in LV performance and functional capacity, respectively. RESULTS After RV septal upgraded, there was a significant decrease in paced QRS duration (171.2 +/- 3.9 ms to 160.4 +/- 3.5 ms, P = 0.0016), increase in LV ejection fraction (55.2 +/- 2.6% vs 60.4 +/- 2.9%, P = 0.0002), the peak ventricular filling rate (2.60 +/- 0.13 s(-1) vs 3.01 +/- 0.14 s(-1), P = 0.046), and 6-MHW (308.2 +/- 31.6 m vs 355.5 +/- 34.2 m, P = 0.015) at 18 months compared with baseline. No changes in these parameters were observed in the control group (P > 0.05). CONCLUSION RV septal pacing upgraded improves LV systolic and diastolic function and functional capacity in patients with previously permanent RV apical pacing. These findings suggest that RV septal pacing can reverse the deleterious effects of RV apical pacing in patients who required permanent ventricular pacing.