Interleukin (IL)-18 is a proinflammatory cytokine which plays a crucial role in T helper (Th)1 type immune response. The aim of this study is to investigate the relationship of serum levels of IL-18 with disease activity and clinical presentations in patients with Behcet's disease (BD). Sixty patients with BD and 20 healthy controls were included in the study. Patients were grouped as having active or inactive disease according to the Leeds activity score. They were also separated as a systemic involvement or mucocutaneous symptoms only. Patients with systemic involvement were further grouped according to the presence of ocular, articular and vascular involvement. IL-18 levels were significantly higher in all patient subgroups as compared to healthy controls and found to be correlated with the activity score in patients having active disease. In conclusion, this cytokine participates in the pathogenesis of BD and its levels are correlated with the disease activity. Detection of increased levels of IL-18 in patients with inactive disease implies that Th1 activation and subclinical inflammation persist during the inactive period of the disease.