Being a high-molecular-weight adhesive glycoprotein, fibronectin (Fn) is suggested to be a component of immune complexes and may participate in the clearance of immune complexes. In Behçet's disease (BD), a multisystem disorder of unknown etiology, immune complexes have been shown to be deposited in affected tissue during disease activity, suggesting an immune mechanism. This study investigates the relationship between Fn and circulating immune complexes (CIC) and evaluates the changes in the levels of Fn and CIC along with disease activity. In 63 patients (31 active, 32 inactive) with BD, plasma Fn and serum CIC, C-reactive protein (CRP), erythrocyte sedimentation rate (ESR) and the third and fourth components of the complement system (C3, C4) were studied. The mean ESR, CRP, C3 and C4 levels of active BD patients were found to be significantly higher than those of the inactive BD patient group. Although the mean Fn and CIC levels of BD patients were not significantly different from those of the healthy control group, Fn and CIC values of active BD patients were significantly lower than in the inactive group. Moreover, no significant correlation was observed among Fn levels and ESR, CRP, C3, C4 and CIC levels in BD patients. The result of this study suggest that the variation in Fn concentration is independent of the acute-phase response. The lack of relationship between the CIC and Fn concentrations indicates that IC deposition in the vessel wall is independent of the CIC levels. In order to determine the exact roles of Fn and IC, further studies in tissue specimens are required.