Cystic fibrosis (CF) is a genetic disorder that affects not only pulmonary function but also multiple organ systems. The fl ow-mediated dilation (FMD) test is a noninvasive assessment of endothelial function and nitric oxide bioavailability. Thus, the purpose of this study was to determine (1) whether endothelial dysfunction is present in young patients with CF and (2) whether endothelial function is associated with pulmonary function and exercise capacity. Fifteen patients with CF and 15 demographically matched control subjects participated in this study. Spirometry, brachial-artery FMD, and a maximal exercise capacity test on a cycle ergometer were performed on all subjects to determine pulmonary function, endothelial function, and exercise capacity, respectively. No differences ( P . .05) in age, height, or BMI were observed between patients with CF and control subjects. FEV 1 (% predicted), FEV 1 /FVC, and forced expiratory fl ow between 25% and 75% of vital capacity were lower in patients with CF. Volume of oxygen consumption peak (absolute and relative) was similar between groups; however, volume of oxygen consumption (% predicted and mL/kg fat-free mass/min) and peak workload were significantly ( P , .05) lower in patients with CF. FMD (4.9% 2.6% vs 7.5% 3.1%; P 5 .018) was lower in patients compared with control subjects, respectively. Relationships between FMD and both pulmonary function and exercise capacity were identified. For the fi rst time to our knowledge, these data provide evidence of vascular endothelial dysfunction in a fairly healthy cohort of young patients with CF. In addition, our data demonstrate the complex relationships between endothelial function and both pulmonary function and exercise capacity in young patients with CF.