The possibility of abnormal platelet function in patients with tension headache was evaluated with serial plasmatic measurements of serotonin, beta-thromboglobulin and platelet factor 4. In 40 patients with tension headache we found increased serotonin, thromboglobulin and platelet factor 4 levels during the painful phases as compared with the asymptomatic phases. These data might indicate that an increased platelet aggregation plays some role in the pathogenesis of tension headache.