Behcet's disease (BD) has been known for many years, yet the etiology of the systemic vasculitis remains unknown. Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide (NO) synthase. ADMA is involved in endothelial dysfunction in various vascular diseases and its level in BD is unclear. This study was performed to evaluate the relationship between ADMA and NO levels in plasma of patients with BD. There were 3 groups of 30 subjects: (a) controls, (b) BD patients with mucocutaneous involvement, and Q BD patients with vascular involvement. Plasma NO levels were assayed by spectrophotometry and plasma ADMA levels were assayed by an ELISA test. Plasma ADMA levels were higher in both groups of BD patients than in the controls; the ADMA levels were higher in the BD patients with vascular involvement than in the mucocutaneous group. Plasma NO levels were lower in both groups of BD patients than in controls; plasma NO levels were lower in the BD patients with vascular involvement than in mucocutaneous group. In the combined groups of 60 BD patients, there was significant inverse correlation between the plasma concentrations of ADMA and NO (r = -0-570, p < 0.001). Plasma lipid profiles did not differ significantly between the BD patients and the controls. These results are evidence for increased plasma ADMA levels and decreased plasma NO levels as risk factors for cardiovascular events in BD patients. Inhibition of NO synthesis by ADMA may contribute to vascular involvement in BD.