To investigate the pathophysiology of muscle contraction headache (MCH), several parameters of inflammation were evaluated in 48 patients suffering from MCH, and compared with 50 healthy control subjects: erythrocyte sedimentation rate; C-reactive protein; white blood cell count; immunoglobulin (Ig) G, A, M; and complement component 3(C3) and C4. Serum C3 and C4 levels were significantly elevated in MCH as compared to control subjects, but the other parameters did not differ significantly. This suggests that the inflammatory process may be involved in the production of MCH.