Background: Early elevations in the parenchymal signal intensity on T2-weighted images, including fluid-attenuated inversion recovery (FLAIR) sequences, within the ischemic territory are considered as a marker of successful recanalization following thrombolytic treatment. In this study, our aim was to assess whether quantitatively determined FLAIR hyperintensity increases could be predictive of improved functional outcome in patients with acute ischemic stroke. Methods: Patients receiving intravenous thrombolysis for proximal anterior circulation strokes were included in the study. FLAIR hyperintensity ratio was determined on magnetic resonance imaging obtained within 72 hours of symptom onset. Univariate and multivariate analyses were performed to determine predictors of good functional outcome at 90 days. Results: The study population was composed of 65 patients. The median (interquartile range) FLAIR hyperintensity ratio was significantly higher among patients with good functional outcome (modified Rankin Scale score <= 3 at day 90, 1.4 [1.2-1.7] versus 1.2 [1.1-1.4], P = .005). Patients with a FLAIR hyperintensity ratio of 1.3 or higher were 4.4 (95% confidence interval 1.6-12.7) times more likely to be independent functionally at the end of 3 months. Higher admission National Institutes of Health Stroke Scale score and age, together with lower FLAIR hyperintensity ratio (P = .006), were found to be significantly and independently related to unfavorable outcome at 90-day follow-up in multivariate analyses. Conclusions: Our findings suggest that a rise in FLAIR hyperintensity signal within the ischemic tissue is suggestive of favorable outcome in patients undergoing intravenous thrombolysis. This tissue marker of favorable outcome is irrespective of other parameters that are crucial in the prognosis of ischemic stroke, such as age and stroke severity.