Background: It is known that initial septal activation occurs on the right (rather than on the left) septal surface in left bundle branch block (LBBB). The purpose of this study was to evaluate the characteristics of ventricular septum and ventricular dyssynchrony in patients with LBBB. Methods: Fifteen patients with LBBB and non-ischemic dilated cardiomyopathy (LVEF<35%, LBBB-DCM) and 9 patients with LBBB (LVEF>50%, LBBB-NL) were enrolled. Eleven patients with narrow QRS complex were included as control. Color-coded tissue Doppler images were obtained from apical 4-chamber view for longitudinal strain (琯) in each wall. Peak 琯 was measured during a cardiac cycle and the time interval from QRS onset to peak 琯 was obtained in left and right side ventricular septum(Ls and Rs), LV lateral (LL) and RV free (RF) walls at the basal-mid level. Intra-ventricular dyssynchrony index was defined as the difference of the time interval (TD) between LL and Ls. Inter-ventricular dyssynchrony index was defined as the TD between LL and RF, and between Ls and Rs. Results: In patients with LBBB, peak 琯 of Ls was significantly reduced, but peak 琯 of Rs was preserved (Table). Compared with controls, patients with LBBB had significant intra- and inter-ventricular dyssynchrony (all p<0.05). Five patients with LBBB-DCM had single large peak 琯 of Ls from the isovolumic contraction period and very large intra-ventricular dyssynchrony index (399.8짹94.2ms, p=0.001) and inter-ventricular dyssynchrony index (TD(Ls-Rs), 293.0짹168.4ms, p=0.03). In contrast, the other 10 patients had two negative 琯 of Ls with first small 琯 during isovolumic contraction period and second large peak 琯. These patients showed smaller TD (Ls-Rs) and not larger TD(LL-Ls) than in patients with LBBB-NL (p=0.005 and p=0.83, respectively). Conclusion: Patients with LBBB have both significant intra- and inter-ventricular dyssynchrony and have two different function of Ls and Rs. Inter-ventricular dyssynchrony within the septum is significantly exaggerated or rather decreased in patients with LBBB-DCM, which may be provided clues for the non-responders to cardiac resynchronization therapy because this has dual pacing usually on the LV lateral wall and right side of ventricular septum and may rather induce significant ventricular dyssynchrony.