Acute necrotizing myositis is described in a 22-yr-old man with clinically diagnosed Behçet's disease. Light microscopic examination revealed a prominently granulocytic-monocytic infiltration of the muscle with severe necrosis. An infectious (bacterial, fungal or parasitic) etiology could be excluded by specific staining techniques and by immunohistochemistry. Vascular deposition of immune complexes was detected by direct immunofluorescence. Electron microscopy revealed severe structural damage and phagocytosis of muscle fibers. In the endomysium, leukocytes and occasional erythrocytes were found. Virus-like particles were not seen. The relevant literature on muscular involvement in Behçet's disease is reviewed. It is suggested that two different stages of inflammation occur in Behçet's disease. In the acute stage it presents as a granulocytic-monocytic necrotizing reaction developing from a neutrophil-mediated vasculitis. In the later phase lymphocytic infiltrations predominate. Despite the rare involvement of muscles the diagnosis of Behçet's disease should be considered particularly in younger patients presenting with muscular symptoms like pain and swelling pre-dominantly of the lower extremities.