To discriminate between the general analgesic effect of acetylsalicylic acid (a.s.a.) and a specific effect by inhibition of platelets we treated 27 migraine patients with low doses of acetylsalicylic acid. In a double-blind randomized cross-over study a small change in number of migraine attacks was found which however must be ascribed to a period effect. Severity of attacks as judged by the patients was similar in the placebo and drug periods. The frequency of attacks and the effect of a.s.a. were not significantly different in patients with platelets relatively sensitive to ADP and those with a slow response to ADP. No correlation was found between number of attacks and inhibition of the ADP aggregation obtained by administration of a.s.a. We concluded that low doses of a.s.a. are not useful to reduce the number or the severity of migraine attacks, and that no correlation exists between migraine attacks and a.s.a. effects on ADP-induced platelet aggregation. High doses of a.s.a. appear to be beneficial in migraine only by a general analgesic effect.