Background: Left bundle branch block (LBBB) is an ECG finding suggesting a worse prognosis in cardiovascular diseases. The aim of this study was to elucidate the relation between QRS duration and left ventricular (LV) function in patients with LBBB. Methods: 95 patients (M:F=35:60, mean age 69짹13 yrs) with LBBB on ECG was recruited. In these subjects, the relations among QRS duration, the demographic findings and the conventional echocardiographc findings including LV systolic and diastolic function were analyzed. Results: 1.The QRS duration was closely related to LV ejection fraction (p<0.005, r= -0.613)(Figure), LV end-systolic volume(p<0.005, r=0.418), LV end-diastolic volume (p=0.029, r=0.224), mitral E wave velocity (p=0.041, r=0.210), mitral A wave velocity (p=0.030, r=-0.223), mitral E/A ratio(p=0.006, r=-0.278). 2. The other parameters such as age, gender, the presence of diabetes mellitus and hypertension were not related to QRS duration. 3. The QRS duration ��� 158msec showed a sensitivity of 73% and a specificity of 77% for predicting of LV EF ��� 35%. Conclusions: The QRS duration is an important predictor reflecting the disease progression in patients with LBBB. Therefore the QRS duration should be closely monitored in symptomatic patients with LBBB.