Several disturbances in platelet function have been reported in migraine and tension-type headache (TH). The plasma 11-dehydrothromboxane B 2 (11-dTXB 2 ) is free from artifactual increase during blood sampling, and it can be a reliable indicator of thromboxane A 2 (TXA 2 ) production in vivo. TXA 2 is a very potent proaggregatory and vasoconstrictory metabolite formed in the platelets. We investigated plasma 11-dTXB 2 and 5-hydroxytryptamine (5-HT) levels in patients with migraine during headache-free periods and in patients with chronic TH. The mean value of plasma 11 -dTXB 2 levels in migrainous patients was significantly higher than those in TH patients and healthy controls. The mean value of plasma 5-HT levels in TH patients was significantly lower than those in migrainous patients and healthy controls. There was no correlation between plasma 11-dTXB 2 levels and plasma 5-HT levels in any group. The results suggest the existence of continuous platelet activation in migrainous patients.