Background: Right ventricular pacing(RVP) makes dyssynchronous myocardial movements mimicking the heart with left bundle branch block(LBBB), which is known to be associated adverse cardiovascular outcome. We compared the characteristics of left ventricular synchrony and function between the RVP and LBBB Methods and results: Twelve patients with RVP(DDD type) which continuously paced right ventricle more than 95% of total heart beats and 5 patients with non-ischemic LBBB were enrolled. (Mean age; 70짹9.4 vs 75짹7.9, p=0.521). Color coded tissue Doppler imaging was recorded from apical four, three and two chamber views of left ventricle (LV). The time interval from the onset of QRS complex on ECG to the peak systolic velocity(Ts) during ejection period, early diastolic velocity(Te), and second peak positive velocity (Tp2) after the closure of aortic valve (over the isovolumic relaxation time,IVRT) were measured from tissue Velocity Imaging(TVI). LBBB group had larger LV size and lower ejection fraction(LVEF) than RVP group (LV end-diastolic dimension; 58.9짹7.3 vs 45.9짹6.6mm, p=0.04, LV end-systolic dimension; 45.1짹7.9 vs 27.3짹6.7mm, p=0.002, LVEF; 32짹9.7 vs 53짹11.2%, p=0.004). QRS duration was wider in RVP group.(172.0짹10.9 vs 151.2짹16.2msec, p=0.027) When left ventricular synchrony was assessed by the maximal difference of Ts and Te, there was no significant difference between RVP and LBBB group (��Ts 110.3짹37.8 vs 99.7짹76.4 ms, p=0.703, ��Te; 71.1짹29.8 vs 101.6짹71.2, p=0.221, respectively). However, Tp2 was significantly different between two groups (66.1짹31.1 vs 125.6짹36.8msec, p=0.004) Conclusion : There was marked dispersion of LV myocardial movements over IVRT in LBBB group compared to RVP group although no significant difference in systolic or diastolic synchrony. Isovolumic relaxation is an energy consuming active process of LV and dyssynchronous movement of ventricular myocardium in this period may represents intrinsic abnormality of cardiac muscle with LBBB, which is not observed in RVP group.