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Aims Left ventricular (LV) endocardial pacing may address the limitations in the selection of an LV pacing site and provide improvements in cardiac resynchronization therapy (CRT) effectiveness. We report on the feasibility, the safety, and the short-term outcome of a leadless ultrasound-based technology for LV endocardial resynchronization in heart failure (HF) patients enroled into the Wireless Stimulation Endocardially for CRT (WiSE-CRT) study. Methods and results Seventeen HF patients were enroled and categorized as: (i) patients in whom attempted coronary sinus lead implantation for CRT had failed ( n = 7); (ii) patients with a previously implanted CRT device, not responding to CRT ( n = 2); and (iii) patients with previously implanted pacemakers or implantable cardioverter-defibrillator and meeting the standard indications for CRT ( n = 8). System implantation was achieved in 13 patients (76.5%); mean R-wave amplitude was 5.6 ± 3.2 mV and the mean pacing threshold was 1.6 ± 1.0 V, respectively. In one patient, no sufficient pacing thresholds were found; in three patients pericardial effusion occurred. Biventricular pacing was recorded in 83% and 92% of the patients at 1 month and 6 months, respectively. QRS duration was shorter during biventricular pacing compared with right ventricular pacing at 1 month (−41 ms; P = 0.0002) and 6 months (−42 ms; P = 0.0011), respectively. At the 6-month follow-up, two-thirds of the patients had at least one functional class change. Left ventricular ejection fraction significantly increased ( P < 0.01) by 6 points at the 6-month follow-up. Conclusion The feasibility of providing an endocardial stimulation for CRT with a leadless technology was successfully demonstrated. Despite the promising results for a novel technology, further study is required to definitively conclude the safety and the performance of the system. Clinical Trial Registration Information NCT01294527.