Abstract： Background Using tissue Doppler imaging and conventional echocardiographic technique, we examined the cardiac function and synchronicity in individuals with isolated right bundle branch block (RBBB) or left bundle branch block (LBBB) and assessed the relationship between QRS duration and synchronicity.Methods Subjects with isolated RBBB (n=20), LBBB (n=10) and normal controls (n=20) were studied with conventional echocardiography and tissue Doppler imaging. The difference between aortic and pulmonary preejection intervals was defined as interventricular delay. Parameters in septum and lateral wall were measured using tissue Doppler imaging, including peak sustained systolic velocity (SM), peak early (EM) and late (AM)diastolic velocities as well as time to peak velocities (Ts, TE and TA).Results Subjects with LBBB had lower SM and longer Ts than did the RBBB and control groups (P＜0.05,P＜0.001 respectively). A significant difference was observed in EM, being the lowest in the LBBB and the highest in the control group (P＜0.05). Moreover, TE was longer in the LBBB group compared with the other two groups (P＜0.001). Both AM and TA were similar among three groups (P＞0.05). In the bundle branch block groups, one ventricle lagged about 40 ms behind the other. A significant correlation was found between interventricular delay and QRS duration (r =0.713, P＜0.001).Conclusions Cardiac ventricles were not well synchronized with one ventricle lagging about 40 ms behind the other in subjects with LBBB or RBBB, even though only LBBB group showed barely perceptible, impaired cardiac function. In addition, QRS duration and cardiac asynchronicity were positively correlated.