Background: Left-ventricular (LV) dyssynchrony could be measured by gated SPECT myocardial perfusion imaging (MPI). This study examined the relation between the degree of dyssynchrony and outcome in patients with implantable cardiac defibrillators (ICDs). Methods and Results: We studied 70 patients with ICD and LV ejection fraction (EF) <.40 by gated MPI (performed within 6 weeks of the device implantation). The images were re-processed using phase analysis to derive phase standard deviation (SD) and histogram bandwidth. All-cause mortality and appropriate ICD shocks were identified as the primary endpoint. There were 87% men, aged 62 ± 11 years. The EF was 26 ± 8% (range 12%-39%). The phase SD was 51° ± 20° (range 12°-99°) and the histogram bandwidth was 157° ± 72° (range 21°-327°). The SD and bandwidth were significantly greater than corresponding values in patients with normal EF (15.8 ± 11.8° and 42.0 ± 28.4°, respectively, P < .0001, each). At 1 year, 8 patients (11%) died or had shocks. The patients with events had higher phase SD than those without events (60 ± 5° vs 50 ± 21°, P = .002). The histogram bandwidth was also higher in those with events (185 ± 37 vs 154 ± 75, P = .07). All patients with event had a phase SD ≥ 50°, while none of the patients with a phase SD < 50° (N = 26) had an event (P = .02). Conclusions: The severity of LV dyssynchrony by phase analysis in patients with LV dysfunction, and ICD is associated with increased risk of death and appropriate ICD shock; a phase SD < 50° was associated with no events at 1 year. © 2010 American Society of Nuclear Cardiology.