QRS widening has important clinical and prognostic implications in patients with chronic heart failure. Ventricular conduction abnormalities such as a left bundle branch block, cause ventricular dysynchrony and several hemodynamic disadvantages. The presence of ventricular dysynchrony results in abnormal wall motion, impaired ventricular contractility, decreased ventricular filling, and increased mitral regurgitation. Biventricular pacing has been recently proposed as an adjunct therapy for advanced heart failure in patients with ventricular conduction abnormalities. Biventricular pacing acutely increases the + dP/dt of left ventricle, the systolic blood pressure and the pulse pressure, prolongs the diastolic left ventricular filling time, shortens the mitral regurgitation duration, and reduces the pulmonary wedge pressure. The implantation of biventricular pacemaker results in improvements of the functional class, exercise capacity, quality of life, echocardiographic findings, and neurohormonal data. Although the indication for biventricular pacing has not yet established, patients with functional class III or IV and left bundle branch block or left ventricular conduction delay showing QRS duration > or = 150 ms are good candidates.