Levels of immune complexes (IC) were determined by the Raji-cell and C1q-binding radioimmunoassays in the sera of eighteen patients with Behçet's disease. Eight patients (44%) were found to have significantly elevated levels of IC (range 56 to 1600 microgram equivalent aliquots of heat aggregated IgG (AHG/ml) by the former test and nine by the latter (7 to 500 microgram equivalent AHG/ml). The presence of IC showed a significant correlation with disease activity score (P = 0.003, Mann-Whitney Rank Sum Test). Abnormal values of IC tended to remain abnormal when sera were retested after 1 year. There was no correlation between IC and duration of disease or any specific organ involvement. The IC were found predominantly in fractions of about 19S and greater when fractionated by sucrose density gradient or Sephadex column techniques. The results suggest the possibility that IC may contribute to the pathophysiology of Behçet's disease.