Behçet's disease (BD) is a systemic vasculitis. Although its clinical characteristics are well defined, the etiology and immune pathogenesis are not clear yet. Neutrophilic vasculitis, which is a consequence of immunological events, is suggested as the underlying pathophysiological mechanism. Adenosine deaminase (ADA) is a non-specific marker of T-lymphocyte activation. A total of 75 patients with BD (45 women and 30 men) and 25 age-matched and gender-matched healthy control volunteers (13 women and 12 men) were included in this study. BD patients were divided into three groups according to their clinical findings: inactive BD patients (group 1, n=25); active BD patients under colchicine treatment (group 2, n=25); and active BD patients without colchicine treatment (group 3, n=25). Plasma ADA (p-ADA) levels of all BD patients and the control group were measured and compared. The relationship between p-ADA levels and disease activity, erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) levels was evaluated and correlated. Patients with BD had significantly higher p-ADA levels (20.6+/-6.3 U/l) than control subjects (12.8+/-1.8 U/l; P<0.001). The p-ADA levels of patients with active BD were significantly (for each, P<0.05) higher than those of inactive BD patients or controls. On the other hand, the difference was not significant (P>0.05) between active patients with or without colchicine use. In addition, there were significantly positive correlations between p-ADA, ESR and CRP levels in patients with BD (for each, P<0.05). However, disease duration or haemoglobin levels were not relevant. ADA level may be a valuable and supportive indicator of disease activity and is not affected by colchicine therapy in BD.