Prolongation of QRS (> or =120 ms) occurs in 14% to 47% of heart failure (HF) patients. Left bundle branch block is far more common than right bundle branch block. Left-sided intraventricular conduction delay is associated with more advanced myocardial disease, worse left ventricular (LV) function, poorer prognosis, and a higher all-cause mortality rate compared with narrow QRS complex. It also predisposes heart failure patients to an increased risk of ventricular tachyarrhythmias, but the incidence of cardiac or sudden death remains unclear because of limited observations. A progressive increase in QRS duration worsens the prognosis. No electrocardiographic measure is specific enough to provide subgroup risk categorization for excluding or selecting HF patients for prophylactic implantable cardioverter-defibrillator (ICD) therapy. In ICD patients with HF, a wide underlying QRS complex more than doubles the cardiac mortality compared with a narrow QRS complex. There is a high incidence of an elevated defibrillation threshold at the time of ICD implantation in patients with QRS > or =200 ms. Mechanical LV dyssynchrony potentially treatable by ventricular resynchronization occurs in about 70% of HF patients with left-sided intraventricular conduction delay, a fact that would explain the lack of therapeutic response in about 30% of patients subjected to ventricular resynchronization according to standard criteria relying on QRS duration. The duration of the basal QRS complex does not reliably predict the clinical response to ventricular resynchronization, and QRS narrowing after cardiac resynchronization therapy does not correlate with hemodynamic and clinical improvement. Mechanical LV dyssynchrony is best shown by evolving echocardiographic techniques (predominantly tissue Doppler imaging) currently in the process of standardization.