Background: Left bundle branch block (LBBB) is commonly associated with structural heart disease and left ventricular dysfunction. We propose that the QRS duration and degree of left-axis deviation (LAD) identify significant left ventricular systolic dysfunction in patients with LBBB. Methods: In this prospective study the ejection fraction (EF) of 300 consecutive patients with LBBB was evaluated by echocardiography. The relationship between QRS duration and LAD (axis between -30°and -90°) and EF were derived. Results: There was no significant difference in age, sex, presence of ischemic or nonischemic cardiomyopathy and valvular heart disease, and EF among the patients with or without LAD. The EF of patients with QRS ≥170 milliseconds with LAD (n = 20) and without LAD (n = 18) was 25% ± 16% and 23% ± 13%, respectively (P = .71). The mean EF (24% ± 10%) of the patients with a QRS duration of ≥170 milliseconds (n = 38) was significantly lower than the mean EF (36% ± 16%) of the patients with a QRS duration of <170 milliseconds (n = 262, P < .015). The QRS duration also had a significant (P < .001) inverse correlation with EF (R = 0.37, adjusted R2 = 0.13, SE of estimate = 16.21). However, the QRS axis was not significantly correlated with EF and did not have added predictive value. Conclusions: The QRS duration has a significant inverse relationship with EF and prolongation of QRS duration (≥170 milliseconds) in the presence of LBBB is a marker of significant left ventricular systolic dysfunction. The presence of LAD in LBBB does not signify a further decrease in EF.