The aim of this study was to evaluate the correlation between the change in heart strains and the success rate of Cardiac Resynchronization Therapy (CRT) optimization. We further explored the benefit of speckle tracking for CRT.In this prospective cohort study, CRT-Ds were implanted to 60 patients. 3 months later, the response was evaluated. In the non-responders, optimization based on speckle tracking was performed. The AV interval was optimized with respect to the quality of left ventricle filling and the VV interval was optimized with respect to heart strains. After a further three months, the optimization success was evaluated.Thirty-nine patients responded well to the initial CRT. The response was independent of etiology; the subsequent optimization was however more successful in dilated cardiomyopathy (DCM) (8 out of 9) than in ischemic heart disease (IHD) patients (3 out of 10 responded). The ejection fraction increase and area strain were the best predictors of NYHA improvement.AV and VV optimization in patients who do not respond well to initial CRT seems to have better results in patients suffering from DCM. Speckle tracking (specifically A-strain) may be used to guide CRT optimization (Tab. 2, Fig. 3, Ref. 22).