To study the role of the proaggregatory, vasoconstrictory thromboxane A 2 (TxA 2 ) in migraine, the plasma levels of Thromboxane B 2 (TxB 2 ), the stabile metabolite of TxA 2 , were measured from 14 adults and 8 children with migraine during the attack-free interval. In addition, the platelets' capacity to generate TxB 2 during spontaneous clotting was studied. The plasma TxB 2 concentrations were lowered in the adult migraine patients. Also the platelets' capacity to produce TxB 2 was reduced in migraine children and in adults with classic migraine. The decreased platelet TxA 2 synthesis in the attack-free period of migraine patients may reflect a deficient pool of platelet arachidonic acid, the precursor of TxA 2 , perhaps as a result of increased consumption during migraine attacks.