SYNOPSISMigraine is a disease associated with increased platelet activity.The aim of this paper was to study "in vivo" platelet activation by assessing platelet serotonin (5HT) content and beta-thromboglobulin (B-TG) and platelet factor four (PF4) plasma levels, in headache-free-periods and during migraine attacks.In headache-free-periods, there was no differences in platelet 5HT values between migraine patients and control subjects. On the other hand, there were significant differences in B-TG and PF4 plasma levels. Furthermore, two groups of migraine patients were observed, one with normal B-TG plasma levels and the other with high B-TG plasma levels. PF4 plasma levels behave similarly. During migraine attacks, platelet 5-HT fell, while B-TG and PF4 plasma levels rose, if compared to the values recorded immediately before attacks.We suggest that migraine patients, particularly those ones with high basal values of B-TG and PF4 plasma levels, form a high risk group to cerebrovascular ischaemic diseases. For these patients a prophylaxis with antiplatelet drugs is indicated. (Headache 22:207-212, 1982)