To investigate the association between body mass index (BMI) and the incidence of ischemic stroke in patients with symptomatic artery occlusion, and further to evaluate the utility of BMI as a screening tool for identifying candidates for extracranial-intracranial bypass surgery.We analyzed the relationship between BMI and the occurrence of ipsilateral ischemic stroke (IIS) among patients receiving only medical management in the Carotid or Middle cerebral artery Occlusion Surgery Study (CMOSS). Additionally, we compared the primary endpoint of CMOSS-stroke or death within 30 days, or IIS after 30 days up to two years-among patients with varying BMIs who underwent either surgery or medical treatment.Of the 165 patients who treated medically only, 16 (9.7%) suffered an IIS within two years. BMI was independently associated with the incidence of IIS (hazard ratio: 1.16 per kg/m2; 95% confidence interval: 1.06-1.27). The optimal BMI cutoff for predicting IIS was 24.5 kg/m2. Patients with BMI ≥24.5 kg/m2 experienced a higher incidence of IIS compared to those with BMI <24.5 kg/m2 (17.4% vs. 0.0%, P<0.01). The incidence of the CMOSS primary endpoint was significantly different between the surgical and medical groups for patients with BMI ≥24.5 kg/m2 (5.3% vs. 19.8%, P<0.01) and those with BMI <24.5 kg/m2 (10.6% vs. 1.4%; P=0.02). Surgical intervention was independently associated with a reduced rate of the CMOSS primary endpoint in patients with BMI ≥24.5 kg/m2.Data from the CMOSS trial indicate that patients with BMI ≥24.5 kg/m2 are at a higher risk of IIS when treated medically only and appear to derive greater benefit from bypass surgery compared to those with lower BMIs. Given the small sample size and the inherent limitations of retrospective analyses, further large-scale, prospective studies are necessary to confirm these findings.Copyright © 2024 The Author(s). Published by Wolters Kluwer Health, Inc.