Behcet's disease is a chronic multisystem disorder characterized by a relapsing inflammatory process of unknown aetiology, The increased activity of polymorphonuclear leukocytes (PMN) in Behcet's disease has been intensively studied. PMN elastase, an acute phase reactant, was investigated to determine whether it may serve as a biochemical marker in Behcet's disease. Erythrocyte sedimentation rate, protein electrophoresis, immunoglobulins (IgA, IgG, IgM), complement components (C-3, C-4) as well as PMN elastase were evaluated in 42 patients with Behcet's disease and 40 healthy subjects. The mean PMN elastase levels were found to be 244.2 mu g/l (median 210 mu g/l, S.D. 126.8) in patients with Behcet's disease and 44.3 mu g/l (median 45, S.D, 19.2) in healthy subjects (P < 0.001). In addition, the mean PMN elastase levels were found to be 321.5 mu g/l (median 300, S.D. 117.9) in the acute phase and 159 mu g/l (median 162, S.D. 59.3) in remission (P < 0.001). It was concluded that PMN elastase may be a good biochemical marker for diagnosis and therapy control in patients with Behcet's disease.