Nitric oxide ( NO) is a free radical and serves many functions within the kidney. Excess NO causes glomerular injury. Behcet's disease (BD) is a systemic immunoinflammatory vasculitis, affecting every organ in the body including the kidneys ( subclinic glomerulonephritis). We investigated the role of urinary total nitrite levels (end product of NO) in BD and evaluated whether urinary concentrations were correlated with its plasma levels or disease activity. Thirty-six consecutive Behcet's patients (19 men, 17 women; 35.9 years), and 20 age- and sex-matched healthy control volunteers (12 men, eight women; 33.2 years) were divided into an active (n = 16) and inactive ( n = 20) period. Urinary and serum NO levels (mumol/mg urinary creatinine) were higher in BD patients (4.1 +/- 0.3) than control subjects (1.7 +/- 0.2; P < 0.001). Serum NO levels in Behcet's patients and control subjects were 51.3 &PLUSMN; 9.8 and 21.7 &PLUSMN; 7.3 μmol/ L, respectively (P < 0.001). Active patients had higher urinary NO excretion (4.9 +/- 0.3) than inactive patients (3.3 +/- 0.3; P < 0.01). Urinary NO levels were correlated with its serum levels (r(2) = 0.69, P < 0.001). Higher urinary NO levels found in BD may be produced by the kidney as a result of an inflammatory stimulation. As excess NO is toxic to the tissues, increased NO levels may play a role in mediating subclinic glomerular injury of such patients. However, we could not determine the exact site(s) of NO synthesis by the kidney, such as the glomeruli, blood vessels and/or the tubular cells. Whatever the source, urinary NO levels may be used as a new activity marker in the diagnosis and follow up of BD by serial measurements.