We evaluated patients with end-stage heart failure who have a high likelihood of response to cardiac resynchronization therapy (biventricular pacing). It appears that 20% of patients do not respond to this expensive therapy despite the use of selection criteria (dilated cardiomyopathy, heart failure, New York Heart Association class II or IV, left ventricular ejection fraction <35%, left bundle branch block, and QRS >120 ms). The presence of left ventricular dyssynchrony is needed to result in improvement after cardiac resynchronization therapy.