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Purpose: Recent reports suggest the occurrence of electrical remodeling in patients who reduce ventricular volumes after cardiac resynchronization therapy (CRT). We have performed this pilot study in order to asses this potential relationship.Methods: We included 20 patients with idiopathic dilated cardiomyopathy and indication for CRT. Patients with atrial fibrillation or pacemaker stimulation at baseline were excluded. We performed measurements of native QRS width and ventricular volumes before implantation and at six month follow-up. Ventricular remodeling was defined as an end- systolic volume reduction ≥ 10%. Biventricular pacing was inhibited to record native QRS and electrical remodeling was achieved when a narrowing of the intrinsic QRS width was present.Results: Of the 20 patients included (61 ± 10 years, 40% women), 15 (75%) showed echocardiographic left ventricular reverse remodeling. These patients showed a significant reduction in the intrinsic QRS duration (169 ± 15 vs. 154 ± 12 ms, p = 0.032) compared to the patients without echocardiographic reverse remodeling (180 ± 23 vs. 180 ± 16 ms, p = 0.977). In addition, these patients, who showed similar clinical features, baseline echocardiographic data, and device programming parameters, as the ones who didn't show reverse remodeling at follow up, were characterized by a narrower paced QRS duration achieved with the CRT device (121 ± 15 vs. 146 ± 24 ms, p = 0.021).Conclusions: In patients with idiopathic dilated cardiomyopathy, the reduction in left ventricular end-systolic volume after CRT is associated with electrical remodeling. This phenomenon seems to be determined by the achievement of a narrower paced QRS at implant.