The plasma concentration of the platelet-specific protein beta-thromboglobulin (beta-TG) was measured in 39 normal subjects and 568 patients of neurological diseases. The beta-TG RIA commercially available KIT was also evaluated. Abnormally high plasma levels of beta-TG were demonstrated in groups of ischemic or obstructive cerebrovascular diseases as compared with that of normal subjects. The highest concentrations were found in 8 patients with Moya-Moya disease, (mean concentration of beta-TG was 204.4 ng/ml), completed stroke at an acute stage was next (mean beta-TG level was 194.8 +/− 70.8 ng/ml). On the other hand, many hemorrhagic cerebro-vascular diseases or other neurological diseases such as brain tumors, hydrocephalus, etc. do not show elevated beta-TG levels. In many patients with ischemic or obstructive cerebro-vascular diseases treated with anti-platelet drugs such as Aspirin, Dipyridamole, Bencyclane or Ticlopidine, a significant fall in plasma concentration of beta-TG was chronologically demonstrated. The measurement of plasma beta-TG concentration may be useful not only in the diagnosis of ischemic or obstructive cerebro-vascular disorders but also in judging the efficacy of anti-platelet therapies and prognosis.