Platelet activation is assumed to occur in many patients with strokes of transient ischemic attacks (TIAs). Determination of this activity can now be measured by assaying the platelet-specific proteins, platelet factor 4 and beta-thromboglobulin. These proteins are platelet specific and are released during irreversible aggregation. The plasma level of these proteins was measured in patients with strokes and TIAs and both groups had significantly greater values than healthy control subjects. These markers of platelet activation may provide further understanding of the role of platelets in the etiology, diagnosis, and treatment of cerebrovascular disease.