Background: There are very limited data regarding the effects of blueberry flavonoid intake on vascular function in healthy humans. Objectives: We investigated the impact of blueberry flavonoid intake on endothelial function in healthy men and assessed potential mechanisms of action by the assessment of circulating metabolites and neutrophil NADPH oxidase activity. Design: Two randomized, controlled, double-blind, crossover humanintervention trials were conducted with 21 healthy men. Initially, the impact of blueberry flavonoid intake on flow-mediated dilation (FMD) and polyphenol absorption and metabolism was assessed at baseline and 1, 2, 4, and 6 h after consumption of blueberry containing 766, 1278, and 1791 mg total blueberry polyphenols or a macronutrient and micronutrient-matched control drink (0 mg total blueberry polyphenols). Second, an intake-dependence study was conducted (from baseline to 1 h) with 319, 637, 766, 1278, and 1791 mg total blueberry polyphenols and a control. Results: We observed a biphasic time-dependent increase in FMD, with significant increases at 1–2 and 6 h after consumption of blueberry polyphenols. No significant intake-dependence was observed between 766 and 1791 mg. However, at 1 h after consumption, FMD increased dose dependently to #766 mg total blueberry polyphenol intake, after which FMD plateaued. Increases in FMD were closely linked to increases in circulating metabolites and by decreases in neutrophil NADPH oxidase activity at 1–2 and 6 h. Conclusions: Blueberry intake acutely improves vascular function in healthy men in a time- and intake-dependent manner. These benefits may be mechanistically linked to the actions of circulating phenolic metabolites on neutrophil NADPH oxidase activity. This trial was registered at clinicaltrials.gov as NCT01292954 and NCT01829542.