Objective: To determine the effect of plasma from patients with Behcet's disease (BD) on the production of nitric oxide (NO) in cultured human umbilical vein endothelial cells (HUVECs). Subjects and Methods: NO levels were measured in cell culture media after 24-hour incubation of the cells with plasma obtained from 22 BD patients and 16 age/sex-matched healthy control subjects. After treatment of the patients with colchicine and/or nonsteroidal anti-inflammatory drugs, 12 of the patients were considered to be in the inactive phase of the disease. Levels of NO production were also measured in these 12 patients. In addition to the in vitro experiments, erythrocyte sedimentation rate, alpha(1)-antitrypsin, alpha(2)-macroglobulin and neutrophil counts were measured in the patients and controls. Results: Levels of NO in active state BD patients (15.9 mumol/10(6) cells) were significantly lower than values obtained from both patients in the inactive period (19.2 mumol/10(6) cells) and the control group (19.7 mumol/10(6) cells). No significant differences were observed in induced NO products between the patients in the inactive stage and control subjects. Conclusion: Plasma from BD patients decreased the level of NO production in the HUVECs, and therefore may cause dysfunction in the endothelial NO synthase activity. Copyright (C) 2004 S. Karger AG, Basel.