Cardiac resynchronization therapy (CRT) assists patients with advanced heart failure (HF) by improving left ventricular (LV) dyssynchrony, but there are significant numbers of non-responders, 1 reason being that the QRS duration is used as the only surrogate determinant of mechanical dyssynchrony, so an effective indicator of LV dyssynchrony is required. The present patient, who had HF, underwent CRT and showed clinical improvement with marked LV reverse remodeling. The regional contraction timing in the LV was assessed with software developed in an application on ECG-gated SPECT myocardial perfusion imaging that depicts the time-volume relationship of the segmented ventricular myocardium and the dispersion of time to end-systole as an expression of dyssynchrony. It was reduced in this patient following CRT. Discordance of systole in regional myocardial segments may present as mechanical dyssynchrony in the LV and could be used as an alternative to QRS duration. Quantitative assessment of dyssynchrony may be possible using this novel method, but further evaluation of the methodology is required. (Circ J 2009; 73: 1550 - 1553)