Primary aldosteronism (PA) is a secondary hypertension characterized by autonomous aldosterone hypersecretion from adrenocortical adenoma and/or hyperplasia. Recently it has been suggested that aldosterone excess is directly involved in the development of cardiovascular injury in PA independent of its hypertensive effect. The present study was designed to examine the relationship between aldosterone excess and endothelial dysfunction in PA patients. 25 PA patients were studied for vascular endothelial function by ultrasound measurement of flow-mediated vasodilation (FMD), and 10 PA patients were re-evaluated 3 months after surgical or medical treatment; 10 age-, gender-, and blood pressurematched hypertensive patients served as control subjects. Percent (%) FMD in PA patients (4.6+/-2.0%) was significantly (p < 0.0001) lower than that in the control subjects (7.9+/-2.0%). %FMD showed significant (p < 0.05) negative correlations with systolic blood pressure (SBP) (r=-0.48), brachial-ankle pulse wave velocity (r=-0.52), plasma aldosterone concentration (PAC) (r=-0.42), and aldosterone-renin ratio (ARR) (r=-0.42), while SBP showed a positive correlation with PAC (r=0.47). Percent FMD, SBP, PAC, and ARR significantly (p < 0.05) improved after surgical and medical treatment, although the changes of %FMD did not correlate with those of SBP, PAC or ARR. In conclusion, the present study has demonstrated that PA patients have endothelial dysfunction, which is related to aldosterone excess and raised blood pressure, and reversible after treatment, suggesting that aldosterone excess contributes to the development of endothelial dysfunction due to its hypertensive effect and/or its direct effect on the cardiovascular system.