Objective: Examine the trajectory of left ventricular ejection fraction (EF) among patients eligible for implantable cardioverter-defibrillator (ICD) therapy Background: Ejection fraction is the cornerstone criterion for ICD therapy, but the risk of sudden cardiac death (SCD) remains after an improvement in EF. Methods: We examined the trajectory of EF among 1178 participants of the Sudden Cardiac Death in Heart Failure Trial (SCD-HeFT) who had 3 or more assessments of EF, at least 90 days apart. A follow-up EF >35% or >10% absolute increase in EF from baseline were examined as the criteria for EF improvement. Results: At first follow-up, 381 (32%) patients had an improvement of EF to >35%. However, EF had returned back to 35% in 109 (27%) of these patients at second follow-up. Similarly, 446 (38%) patients experienced a >10% improvement in EF at first follow-up, but 109 (24%) of these had a subsequent >10% decrease in EF at the second follow-up. Of the 32 patients with normalized EF (≥55%) at first follow-up, 18 (56%) had a subsequent >10% decrease in EF. The fluctuation in EF was present in both ischemic and non-ischemic cardiomyopathy but a higher proportion of patients with non-ischemic cardiomyopathy had an improvement in EF to >35% at first follow-up compared to those with ischemic cardiomyopathy (38% vs. 27%, p=<0.0001). Conclusion: There is substantial fluctuation of EF among patients who are eligible for ICD therapy. These data may help explain the continued risk of SCD after improvement in EF.