This study was designed to examine the effect of left bundle branch block (LBBB) on systolic and diastolic function of the left ventricle (LV) in patients with heart failure and in normal subjects. Thirty-six patients with heart failure and LBBB (group I), 36 patients with heart failure with normal conduction (group II), and 41 subjects with isolated LBBB (group III) were compared. Coronary angiography was performed and LV end diastolic pressure was calculated. Echocardiography was performed on all patients. LV ejection fraction and mean rate of circumferential shortening were calculated. The following Doppler parameters were evaluated: peak rapid filling velocity (E wave), peak atrial filling velocity (A wave), E- and A-wave integrals, E-wave acceleration time and deceleration time (EDT) and rates (EAR and EDR), the E/A ratio and its integral, and diastolic flow time (DT). The ejection time, isovolumetric relaxation time (IRT), and preejection period were measured using the aortic and mitral flow, LV end diastolic pressure was calculated as 28 ± 4 mm Hg, 22 ± 5 mm Hg, and 15 ± 3 mm Hg in groups I, II, and III, respectively. Although the systolic function parameters in group III patients were different, the diastolic function parameters of group II were found to be quite similar to those of group III patients. Comparison of group I patients with group II patients showed that there was a similarity between LV systolic function parameters while the diastolic function parameters were different (E/A, p = 0,004; EAR, p < 0.001; EDR, p < 0.001; EDT, p < 0.001; IRT, p = 0.024; DT, p = 0.03). In conclusion, this study evaluating the effects of LBBB in normal subjects (isolated LBBB) and patients with heart failure showed that LBBB causes diastolic function impairment in normal subjects similar to those of patients with heart failure, and also increases impairment of diastolic function in patients with heart failure.