In 14 haemodialysis patients, platelet count, secondary platelet aggregation rate, immunological antithrombin III and antithrombin III activity were lower and plasma beta-thromboglobulin higher than in 14 age- and sex-matched controls. In contrast, primary platelet aggregation, the degree of secondary aggregation and circulating platelet aggregates did not differ. Haemodialysis was associated with signs of platelet damage reflected by increase in plasma beta-thromboglobulin, extraction of platelets in the dialyser and decline in platelet count. Platelets in the dialyser effluent line were less aggregable than platelets in arterial blood. Circulating platelet aggregates and immunological antithrombin III were unchanged during dialysis whereas antithrombin III activity showed a minor rise. In conclusion, uraemics show a decreased rate of secondary platelet aggregation, and haemodialysis confers further platelet injury due to blood/surface interactions during extracorporeal circulation. The defective platelet function and low antithrombin III activity may help to explain the paradoxical occurrence of both haemorrhagic and thrombotic complications in uraemia.