Background and Purpose: Cerebral autoregulation is impaired in patients with acute ischemic stroke. The purpose of this study was to investigate whether dynamic cerebral autoregulation (dCA) indices constitute an independent functional outcome predictor of acute ischemic stroke. Methods: In this study, 86 patients at days 3 to 7 after acute ischemic stroke and 40 age- and sex-matched controls were enrolled for assessing their dCA indices under spontaneous hemodynamic fluctuations. The dCA indices of patients with favorable outcomes (modified Rankin Scale score ≤1 at 3 months, n=65), patients with unfavorable outcomes (modified Rankin Scale score ≥2 at 3 months, n=21), and controls were compared. Results: The dCA indices, namely the phase shift at very low frequency band (Phase-VLF), in the patients with unfavorable outcomes were significantly worse than those in the patients with favorable outcomes. However, the Phase-VLF in the patients with favorable outcomes did not differ from those in the controls. Impaired dCA was associated with elevated mean arterial pressure and large infarction volume but was also present in patients with normal mean arterial pressure or small infarction volume. Phase-VLF was a predictor of outcomes in the receiver operating characteristic analysis (area under the curve: 0.722; P<0.001). Multivariate analysis revealed that a Phase-VLF value of <61° was independently associated with unfavorable outcomes (odds ratio=4.90; P=0.024). Conclusions: Phase-VLF is an independent functional outcome predictor of acute ischemic stroke.