Cardiac resynchronization therapy is a novel nonpharmacologic approach to treating patients who have advanced heart failure with left bundle branch block (LBBB). Such a therapy is based on the original theory that synchronous biventricular pacing is able to reduce the interventricular delay caused by LBBB in patients with heart failure. Although there is convincing evidence that biventricular pacing increases the left ventricular ejection fraction, decreases mitral regurgitation, and improves symptoms caused by heart failure, the percentage of nonresponders to such therapy has been described as high as about one third of patients with heart failure having LBBB. Factors responsible for this relatively high prevalence are reviewed, the most important of them probably being left intraventricular dyssynchrony, which can persist after biventricular pacing, notwithstanding right and left interventricular resynchronization. Such a dyssynchrony, as evaluated by tissue Doppler imaging, may be because of the discordance between the site of the left ventricular pacing and the site of the left ventricular delay. Therefore, to characterize the pathophysiologic pattern of LBBB, the investigators suggest an assessment of the electromechanical dysfunction with a noninvasive reliable technique, such as tissue Doppler imaging, which can be repeated after biventricular pacing.