Cardiac resynchronization therapy (CRT) is an effective treatment for chronic symptomatic systolic heart failure with cardiac dyssynchrony, but about one-third of patients do not respond favorably to the therapy. We hypothesized that acute modifications of the coronary sinus electrode pacing cathode movements induced by biventricular pacing, may be related to resynchronization process and consequently may carry predictive power on CRT response. An automated method for 3D reconstruction of coronary sinus lead’s pacing cathode trajectory (3DTJ) throughout a cardiac cycle showed that trajectory’s geometry suddenly changed in responders (R) upon starting of biventricular pacing, becoming less eccentric and more multi-directional. The TRAJECTORIES study is a multicenter observational study aiming at evaluating the clinical value of 3DTJ. Out of 119 patients enrolled, 50 patients have ended follow-up and have been analyzed. Concordance between 3DTJ metrics and response (>15% reduction in LV end-systolic volume at 6-months) was 82% overall (41/50), 91% in R (31/34), 62% in NR (10/16). The proposed 3DTJ metric showed sensitivity = 91%, specificity = 62%; positive predictive value = 84%, negative predictive value = 77%. From our data, 3DTJ seems a promising tool to acutely predict CS pacing site-specific response to CRT. Importantly, its investigational use as an intra-operatory, real-time guidance for selecting LV pacing sites may open new opportunities for CRT patients’ selection and therapy delivery.