The incidence of thromboembolic complications was compared retrospectively in 90 cadaveric kidney allograft recipients treated with cyclosporin and low-dose steroids and the same number of cadaveric kidney allograft recipients treated with azathioprine, antilymphocyte globulin, and high-dose steroids. In the cyclosporin group, 17 thromboembolic complications occurred in 13 patients: 10 pulmonary emboli, 1 renal vein thrombosis, 3 deep vein thromboses, and 3 haemorrhoidal thromboses. In the azathioprine group, the only thromboembolic complication was 1 episode of superficial thrombophlebitis. Haemostatic tests in cyclosporin-treated and azathioprine-treated patients and normal subjects (10 in each group) showed increased concentrations of factor VIII C, fibrinogen, antithrombin III, and protein C in the cyclosporin-treated patients. Adenosine-5'-diphosphate-induced platelet aggregation was also significantly enhanced in the cyclosporin group. The effect of cyclosporin on haemostasis may predispose to thromboembolic complications.