Background: The clinical value of interventricular(V-V) delay optimization in patients with chronic congestive heart failure(CHF) undergoing implantation of a device for cardiac resynchronization therapy(CRT) has not been clearly demonstrated. Methods: RHYTHM II was a single-blind randomized trial including 121 recipients of a device for CRT with cardioverter/defibrillator capabilities(CRT-D) randomly assigned in a 1∶3 ratio to simultaneous(n=30) versus optimized(OPT)(n=91) biventricular pacing. V-V delay was optimized by echocardiography. The study end points were(1) freedom from CRT-D system-related complications and(2) changes between preimplant and 6 months of follow-up in(a) New York Heart Association CHF functional class,(b) distance covered during a 6-minute hall walk, and(c) quality of life(QOL). Results: In the OPT group, the V-V delay ranged from 0 to 80 milliseconds, with 28.4%of patients stimulated at an OPT V-V delay of 0 milliseconds. The overall 6-month survival free of adverse events requiring invasive interventions was 81.8%. In the whole cohort, 6 months of CRT-D was associated with a significant decrease in New York Heart Association class, increase in the distance covered during the 6-minute hall walk, and improvement in QOL(each P< .0001). The effects of CRT-D on these end points were similar in both study groups. Conclusions: Cardioverter-defibrillator capabilities was associated with a significant alleviation of CHF symptoms, increase in functional capacity, and improvement in QOL. The optimization of the V-V delay conferred no additional benefit compared with simultaneous biventricular stimulation. Background: The clinical value of interventricular (VV) delay optimization in patients with chronic congestive heart failure (CHF) undergoing implantation of a device for cardiac resynchronization therapy (CRT) has not been shown clearly. Methods: RHYTHM II was a single-blind randomized trial included 121 recipients of a device for CRT with cardioverter / defibrillator capabilities (CRT-D) randomly assigned in a 1: 3 ratio to simultaneous (n = 30) versus optimized (OPT) (n = 91) biventricular pacing. optimized by echocardiography. The study end points were (1) freedom from CRT-D system-related complications and (2) changes between preimplant and 6 months follow-up in (a) New York Heart Association CHF functional class, (b) Results: In the OPT group, the VV delay ranged from 0 to 80 milliseconds, with 28.4% of patients stimulated at an OPT VV delay of 0 milliseconds. The overall 6-month survival free o f adverse events requiring invasive interventions was 81.8%. In the whole cohort, 6 months of CRT-D was associated with a significant decrease in New York Heart Association class, increase in the distance covered during the 6-minute hall walk, and improvement in QOL (each P <.0001). The effects of CRT-D on these end points were similar in both study groups. Conclusions: Cardioverter-defibrillator capabilities was associated with a significant alleviation of CHF symptoms, increase in functional capacity, and improvement in QOL. The optimization of the VV delay conferred no additional benefit compared with simultaneous biventricular stimulation.