Background Brachial artery reactivity testing (BART)—a surrogate test of microvascular function—predicts cardiac risk in the nonsurgical population and associates it with adverse outcome after vascular surgery. This pilot study investigated BART-derived variables, including flow-mediated dilation (FMD), in preoperative risk stratification for major thoracic surgery. Methods After institutional review board approval, BART was performed in 63 patients before major thoracic surgery. Ultrasonography recorded two-dimensional images and Doppler flow signals of the brachial artery preoperatively at baseline and after induced reactive hyperemia. Variables derived using BART were correlated with preoperative risk factors, established risk scores, and postoperative complications. Results The median preoperative FMD value in patients without postoperative complications was 11.5%. This value was used to delineate all patients into two groups: low (FMD p p = 0.015) and hospital (14.0 ± 3.3 days versus 6.8 ± 0.6 days; p = 0.007) stays. This cutoff point for FMD accurately predicted 71% of the patients with adverse postoperative events, achieving 71.4% (95% confidence interval, 54.7 to 88.2) sensitivity and 48.6% (95% confidence interval, 32.0 to 65.1) specificity. Conclusions Using BART, preoperative microvascular dysfunction can be identified in patients at increased risk for postoperative complications. These data suggest that larger observational studies and studies exploring preoperative optimization strategies aimed at improving microvascular function are warranted.