Background: More information is required on the relationship between electrical and structural reverse remodeling in patients treated with cardiac resynchronization theraphy. Methods: QRS and JT intervals were investigated during different pacing modes before and 3 months after implantation of a device for biventricular(BiV) pacing in 20 patients with severe drug refractory heart failure(with left ventricular ejection fraction< 40% and QRS >120 ms); structural remodeling was evaluated by echocardiography. Results: QRS interval was significantly shortened by BiV pacing both acutely(p=0.002) and at 3 months(p=0.007). No significant change was found in the JT interval. The extent of QRS shortening obtained by BiV pacing showed moderate correlations with the reduction of end-systolic and end-diastolic volumes(r=0.53, p=0.016 and r=0.45, p=0.045, respectively) as well as with increase of left ventricular ejection fraction(r=0.49, p=0.028) at 3 months. The widening of QRS observed during right ventricular(RV)pacing was greater after 3 months of BiV pacing(with respect to acute assessments), suggesting accentuation of pacing-induced electrical dyssynchrony after a period of pacing-induced resynchronization. Conclusion: The extent of QRS shortening induced by BiV pacing appears to correlate with the reverse structural remodeling(in terms of reduction in end-systolic volume). The acute changes and the remodeling process occurring at mid-term in the overall population of CRT-treated patients do not appear to involve the JT interval. A period of pacing-induced resynchronization appears to accentuate the potential for RV pacing-driven electrical dyssynchrony. Background: More information is required on the relationship between electrical and structural reverse remodeling in patients treated with cardiac resynchronization theraphy. Methods: QRS and JT intervals were investigated during different pacing modes before and 3 months after implantation of a device for biventricular (BiV) pacing Results: QRS interval was significantly shortened by BiV pacing both acutely (p = 0.002) and in 20 patients with severe drug refractory heart failure (with left ventricular ejection fraction <40% and QRS> 120 ms) At extent of found in the JT interval. The extent of QRS shortening obtained by BiV pacing mapped moderate correlations with the reduction of end-systolic and end-diastolic volumes (r = 0.53, p = 0.016 and r = 0.45, p = 0.045, respectively) as well as with increase of left ventricular ejection fraction (r = 0.49, p = 0.028) at 3 months. The widening of QRS observed on rig ht ventricular (RV) pacing was greater after 3 months of BiV pacing (with respect to acute assessments), suggesting accentuation of pacing-induced electrical dyssynchrony after a period of pacing-induced resynchronization. Conclusion: The extent of QRS shortening induced by BiV pacing appears to correlate with the reverse structural remodeling (in terms of reduction in end-systolic volume). The acute changes and the remodeling process occurring at mid-term in the overall population of CRT-treated patients do not appear to involve the JT interval. A period of pacing-induced resynchronization appears to accentuate the potential for RV pacing-driven electrical dyssynchrony.