Background: The aim of this study is to investigate cerebral endothelial dysfunction in patients with reversible cerebral vasoconstriction syndrome (RCVS). Methods: We prospectively recruited patients with RCVS, age-matched controls with episodic migraine, and age-matched healthy controls at Samsung Medical Center from Apr 2015 to Jul 2016. All participants underwent transcranial Doppler evaluation, with a breath-holding maneuver, for the evaluation of bilateral middle cerebral arteries (MCAs), posterior cerebral arteries (PCAs), and the basilar artery (BA). The breath-holding index (BHI) was used to measure cerebral endothelium-dependent vasodilation. Follow-up BHIs were recorded in selected patients with RCVS after 3 months. Results: A total of 84 subjects were recruited for this study (n = 28 in each group of RCVS, episodic migraine, and healthy control; mean age, 49.8 years). The RCVS group showed lower BHIs in all basal arteries, in comparison to healthy controls (p < 0.001, 0.009 for bilateral MCAs, p < 0.001 and 0.028 for bilateral PCAs, and p = 0.060 for the BA). Compared to migraineurs, RCVS patients had lower BHIs only in the anterior circulation (p = 0.002 and 0.038 for bilateral MCAs; p = 0.069 and 0.247 for bilateral PCAs; p = 0.120 for the BA). Of the 10 patients who had follow-up BHIs at 3 months, 7 showed complete normalization, while three did not. Conclusions: Cerebral endothelial function is impaired in a widespread distribution in RCVS. Its role in the pathogenesis and clinical outcome of RCVS should be determined in further studies.