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Background: Most studies recruiting patients with acute ischemic stroke ascertain the final functional status at 3 months post symptom onset. The rates and predictors of late functional improvement (>3 months) in acute ischemic stroke patients has not been systemically studied.Methods: We analyzed data from three phase-III randomized multicenter clinical trials that recruited acute ischemic stroke patients within 3 or within 5 hours of symptoms onset. Late improvement was defined by an improvement of 1 or more grade in modified Rankin scale between ascertainments performed at 3 and 12 months post randomization. We analyzed the rates of late improvement in three groups of patients; patients treated with IV (recombinant tissue plasminogen activator [rt-PA], those treated with endovascular treatment, and those not treated with any acute reperfusion treatment. Multivariate analysis was performed to identify predictors’ of late functional improvement.Results: A total of 2555 patients (mean age ± SD; 66.4 ± 12.9 years) with acute ischemic stroke were analyzed.Late functional improvement was observed in 447 (17.5%, 95% confidence interval [CI] 15.9 to 19.2]) of 2555 patients analyzed. The rates of late functional improvement according to acute reperfusion treatment were as follows: 248 (22.4%) of 1106 in IV rt-PA treated; 108 (23.9%) of 452 in endovascular treated; and 91 (19.1%) of 477 in patients without any treatment (p= 0.179). In multivariate analysis, patients with age ≥65 years (odds ratio [OR], 0.53; 95 % CI 0.35 to 0.80; P = 0.002), and severity of baseline neurological deficits with National Institutes of Health Stroke Scale [NIHSS] score at 24 hours ≥ 20 (OR, 0.40; 95 % CI, 0.28 to 0.58; P = <0.001), were inversely associated with late improvement.Conclusions: Late functional improvement occurs in one fourth of acute ischemic stroke patients and should be considered in estimation of functional recovery in future clinical studies.