Endothelial dysfunction is major pathophysiologic mechanism in cardiac syndrome X (CSX), which causes a decrease in plasma nitrite oxide (NO) levels. It was demonstrated that nebivolol improves endothelial function and increases NO release. Despite this pathophysiologic relation, the effect of nebivolol therapy on endothelial function in patients with CSX is unknown. The aim of this study is to evaluate the effect of nebivolol on patients in CSX. Thirty-eight patients who were diagnosed with CSX were prospectively enrolled in the study. The treatment group consisted of 20 patients and the control group consisted of 18 patients. An oral 5-mg dose of nebivolol was given daily and maintained for 4 weeks in the treatment group. Ultrasonographic parameters (brachial artery flow-mediated dilatation [FMD], brachial artery lumen diameters) and inflammatory markers (high-sensitivity C-reactive protein [hsCRP], von Willebrand factor [vWf], and fibrinogen) were measured at baseline and end of the 4 weeks. Brachial baseline lumen diameter, brachial lumen diameter after reactive hyperemia, and FMD were 4.61 +/- 0.49 mm, 4.87 +/- 0.53 mm, and 5.6% +/- 2.3% at baseline. After the nebivolol therapy, there was a significant increase in both brachial artery baseline lumen diameter and lumen diameter after reactive hyperemia (P < 0.001 and P = 0.002). However, there was no significant change in FMD (5.6% +/- 2.2% vs 5.3% +/- 2.1%, P not significant). Levels of hsCRP, vWf, and fibrinogen were significantly decreased (hsCRP: 3.4 +/- 0.49 mg/dl vs 2.97 +/- 0.74 mg/dl, P = 0.001; vWf: 107 +/- 62 vs 86 +/- 58, P = 0.004; fibrinogen: 341 +/- 89 mg/dl vs 299 +/- 87 mg/ dl, P = 0.01) in the treatment group. Nebivolol therapy may have a favorable effect on endothelial function in CSX. Further studies are needed to confirm the clinical significance of nebivolol therapy in CSX.