In vitro abnormalities of platelet behaviour have been described in patients with ischemic cerebrovascular disease (ICD) suggesting that changes in platelet receptors and/or platelet a granules may occur in the circulation. We investigated the frequency of such alterations in 95 patients that were studied within a few days after acute stroke. Using specific antibodies to two intrinsic plasma membrane glycoproteins (GPs), the GPIIb-IIIa complex and GPIV, and to thrombospondin (TSP), a a-granular protein that becomes expressed on the platelet surface upon activation, we were able to distinguish two groups of patients: 16 patients presented an increased concentration of TSP on their platelets demonstrating in vivo platelet secretion. These patients could constitute a group with increased risk of thrombosis. In contrast, 20 patients presented a decreased concentration of GPIIb-IIIa and GPIV. This decrease in intrinsic plasma membrane GPs was associated with a decrease in mean platelet volume and may suggest the occurrence of platelet fragmentation in the circulation.