Patients with Behçet's syndrome have an increased risk of arterial and venous thrombosis, and abnormal platelet function has been implicated. Platelet function was studied in nine patients with Behçet's syndrome and in nine age- and sex-matched healthy volunteers. Platelet aggregation in response to ADP was measured, and the threshold concentration required to produce irreversible aggregation determined. Sensitivity of platelets to the inhibitory effect of prostacyclin was also determined. In addition, plasma levels of the platelet-specific proteins, beta-thromboglobulin and platelet factor 4, and stimulated platelet thromboxane B2 production, were measured. Platelets from patients with Behçet's syndrome showed normal aggregation in response to ADP, irrespective of disease activity. Platelet sensitivity to prostacyclin was, however, decreased compared with controls--with a mean prostacyclin ID50 of 5.5 +/- 1.3 ng ml-1 (mean +/- SEM) and 1.9 +/- 0.3 ng ml-1, respectively (P less than 0.01). This reduction in platelet sensitivity to prostacyclin was greatest in patients with the most active disease. These results suggest that Behçet's syndrome may be associated with altered platelet function, and this may have important consequences with regard to the increased risk of thrombosis associated with this condition.