In order to evaluate the pathogenetic importance of platelet aggregates in cerebrovascular disease, a platelet count ratio method was used to study 66 patients with transient ischemic attacks (TIAs). Thirty normal subjects and 22 patients without thromboembolic disorders were also included as controls. The mean platelet aggregate ratio of the TIA group was 0.75 +/- 0.03 SEM which was significantly lower than that of normal subjects (0.90 +/- 0.02) or patients controls (0.88 +/- 0.01) (P less than 0.01). Seventeen patients with TIA were then treated with aspirin (1,200 mg) and dipyridamole (200 mg) daily. The platelet aggregate ratios were normalized in 13 patients. Of four patients who did not respond to this regimen, one did respond to sulfinpyrazone. When sulfinpyrazone was discontinued, recurrence of symptoms was preceded by an increase in platelet aggregates. These findings suggest that platelet aggregates may play an important role in the pathogenesis of cerebrovascular insufficiency. The determination of platelet aggregates appears useful in selecting patients for antiplatelet therapy.