Background: Patients with functioning renal transplants are at risk of graft thrombosis in the postoperative period, and of fistula thrombosis and other thrombotic events thereafter. Investigation and therapeutic manipulation of haemostasis in these patients offers a means to counter this thrombotic tendency.Methods: Platelet aggregation in whole blood, plasma von Willebrand factor and plasma fibrinogen levels were measured in 32 stable renal transplant patients (creatinine <200 mu mol/litre, age of graft >4 months) and in 32 age, sex and smoking-habit matched normal controls. Results: In both patient and control groups, seven patients were smokers and the remaining 25 were non-smokers. There was no significant difference in age between patients and controls [patients, median: 39 (20-64) years; controls: 38 (24-60) years]. Spontaneous platelet aggregation was significantly higher in the patients at all time points studied [30s-6 min; at 4 min: patients median (interquartile range) 19.4 (11.3-27.3)%; controls, 8.0 (5.1-15.0)%, P < 0.0005]. ADP-induced aggregation was also increased at a concentration range of 0.1-3 mu M (at 1 mu M, 1 min, patients median (interquartile range) was 52.4 (30.5-70.0)%; controls was 16.5 (1.4-31.4)%, P < 0.0001). Transplant patients had significantly higher von Willebrand factor and fibrinogen levels compared with the controls (von Willebrand factor, patients median (range): 158 (13-269)%; controls: 85 (43-223)%, P < 0.00001; fibrinogen, patients: 3.29 (2.12-7.39) g/litre; controls: 2.81 (1.84-4.65) g/litre, P < 0.0002). Conclusion: Patients with stable renal transplants have in vitro evidence of enhanced platelet activation, and increased plasma von Willebrand factor and fibrinogen levels .