Myeloperoxidase was measured in the plasma and in the supernatants of polymorphonuclear granulocyte cultures from patients with Behçet's disease. High levels were found in both plasma and supernatants from patients with active disease. The addition of pentoxifylline to granulocyte cultures determined a significant decrease of myeloperoxidase levels in active patients only. Hyperactive neutrophils are present during the course of Behçet's disease and may be considered of importance in the pathogenesis of the vascular lesions.