BACKGROUND AND OBJECTIVES: Left bundle branch block (LBBB) is an ECG finding that suggests a poor prognosis for patients suffering with cardiovascular disease. The aim of this study was to elucidate the relation between the QRS duration and the left ventricular function in the patients with LBBB. SUBJECTS AND METHODS: 95 patients (M : F=35 : 60, mean age: 69+/-13 yrs) with LBBB observed on ECG were recruited for this study. The relations among the QRS duration, the demographic.clinical findings and the conventional echocardiographic findings, including the LV systolic and diastolic function, were analyzed for these subjects. RESULTS: The QRS duration was closely related to the LV ejection fraction (r=-0.613, p<0.005), the LV end-systolic volume (r=0.418, p<0.005), the LV end-diastolic volume (r=0.224, p=0.029), the mitral E wave velocity (r=0.210, p=0.041), the mitral A wave velocity (r=-0.223, p=0.030) and the mitral E/A ratio (r=0.278, p=0.006). The LV ejection fraction (odds ratio: 0.839[95% confidence interval: 0.751-0.937]) was associated with the supramedian QRS duration (150 msec) according to multivariate logistic regression analysis. The other parameters such as age, gender, the presence of diabetes mellitus and hypertension were not related to the QRS duration. The QRS duration > or =158 msec showed a sensitivity of 73% and a specificity of 77% for predicting a LV EF <35%. CONCLUSION: The QRS duration is an important predictor that reflects the disease progression in patients with LBBB. Therefore, the QRS duration should be closely monitored in symptomatic patients with LBBB.