We studied platelet aggregates, by Wu and Hoak's technique, in 73 patients (49 F and 24 M), aged 14-61 (mean age 50.4 +/- 12.2) with migraine. Platelet aggregates were expressed as Platelet Aggregate Ratio (PAR). We consider as pathological PAR values lower than 86. Mean PAR value of migraine patients (78.3 +/- 8.68 SD) was significantly lower (p less than 0.001) than that of 90 control subjects (95.4 +/- 6.15 SD). Normal PAR values were found only in 27.4% of the migraine patients. The patients were divided according to the interval from the last attack: 17 patients were studied in the 1st week (PAR = 73.76 +/- 7.6 SD; 5.8% of the values in the normal range), 15 in the 2nd week (76.6 +/- 7.02 SD; 13.3% of the values in the normal range) and 41 between 15th and 30th day (80.78 +/- 8.78 SD; 29.6% of the patients in the normal range). The mean PAR value of the patients studied during the first week was significantly lower (p less than 0.01) than that of the patients studied between 15th and 30th day. No significant differences were found between the patients with classical, common and complicated migraine.