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About one third of patients with chronic heart failure have intraventricular conduction delays, which result in dyssynchronous, inefficient cardiac contraction. In several nonrandomized clinical trials, resynchronization via simultaneous pacing of the right and left ventricles has improved cardiac function and quality of life. In this industry-supported, multicenter trial, 453 patients with NYHA class III or IV heart failure, ejection fractions of <35%, and QRS durations of ≥130 msec each received a cardiac-resynchronization device and, then, were randomized to biventricular pacing or to no pacing for 6 months. Standard heart-failure therapy was maintained. Initially, 571 patients were enrolled, but implantation was unsuccessful in 43 patie…