BACKGROUND AND AIMS: Obesity is associated with vascular endothelial dysfunction, which predisposes obese persons to cardiovascular disease. Fat loss in obese persons can lead to metabolic improvements that promote improved cardiovascular health and decrease risk of stroke or myocardial infarction. The objective of this study was to assess the potential of weight loss achieved by bariatric surgery to improve endothelial function. METHODS: Patients scheduled to undergo bariatric surgery were prospectively followed. Biochemical analyses and vascular function testing were performed preoperatively and again at 1, 3, 6, and 12 months postoperatively. FMD and NMD were measured to assess macrovascular endothelial function and ∆ hyperemic flow was measured to assess microvascular endothelial function. RESULTS: Patients (n = 375) ages 41.76 ± 12.35 years, with baseline BMI of 45.64 ± 8.36, experienced a weight reduction of 82.07 ± 33.98 lbs the year following surgery. FMD % increased by 1.28 ± 5.49, NMD % by 4.26 ± 6.23, and ∆ hyperemic flow % by 262.1 ± 519.97. All vascular function variables demonstrated a significant increase over time (p < 0.05) with ∆ hyperemic flow % experiencing the most significant change (p < 0.0001). No difference was found in improvement in vascular function between high (>13 µIU/ml) and low (≤13 µIU/ml) baseline plasma insulin groups (p values all > 0.05). CONCLUSIONS: Bariatric surgery was associated with an improvement in macrovascular and microvascular endothelial function. Further analyses is needed to determine which clinical parameters are optimal predictors of improvements in vascular endothelial function after bariatric surgery.