Background: The American Heart Association/American Stroke Association guidelines strongly recommend a noninvasive intracranial vascular study such as computed tomographic (CT) angiogram in acute stroke patient if endovascular treatment is contemplated. Objective: The objective was to determine the frequency of change in occlusion site between CT angiogram and cerebral angiogram in acute ischemic stroke patients undergoing endovascular treatment. Methods: All acute ischemic stroke patients who underwent a CT angiogram and subsequently underwent endovascular treatment were included. The CT and cerebral angiographic images were reviewed independently to determine presence and location of arterial occlusion. Severity of occlusion was classified by a previously described grading scheme. Clinical outcome at discharge was determined using modified Rankin scale. Results: Computed tomographic angiogram was performed in 150 patients (mean age ± SD, 64.7 ± 16 years) before endovascular treatment. The mean interval (± SD) between CT angiogram and cerebral angiogram was 193 ± 164 minutes, and 65 (43.3%) of 150 patients received intravenous recombinant tissue plasminogen activator before cerebral angiography. Recanalization between CT angiogram and cerebral angiography was seen in 28 (18.7%) patients, whereas worsening of occlusion was seen in 31 (20.7%) patients. We noticed a trend towards higher rates of improvement (60.7% vs 42.0%, P =.07) and favorable outcome at discharge (42.9% vs 28.7%, P =.1) among patients who experienced preprocedure recanalization. After adjusting for age and initial National Institutes of Health Stroke Scale score strata, preprocedure recanalization was not associated with significantly higher rate of favorable outcome (modified Rankin scale, 0-2) at discharge (odds ratio, 2.1; 95% confidence interval, 0.8-5.5). After adjusting for age and National Institutes of Health Stroke Scale score strata, preprocedure worsening was not associated with significantly lower rates of favorable outcomes at discharge (odds ratio, 0.4; 95% confidence interval, 0.1-1.4). Conclusions: A relatively high proportion of patients have preprocedure recanalization or worsening between CT angiogram and cerebral angiogram in acute ischemic stroke patients selected for endovascular treatment.