The incidence of arterial and venous thromboembolic complications was compared in 224 renal allograft recipients who were prospectively randomized and stratified by risk to treatment with either cyclosporine-pred-nisone (CsA-P) (n=117) or azathioprine-prednisone-an-tilymphocyte globulin (AZA-P-ALG) (n=107). Thirteen CsA patients (11%) had 22 thromboembolic events, while 19 AZA patients (18%) had 24 events (P = 0.22). There was no significant difference between the 2 regimens in the number of patients with each type of venous or arterial event or in the number of patients with multiple or lethal events. The incidence of “minor” complications (all except myocardial infarction and stroke) in the related donor subgroup (n=85) and the overall incidence of thromboembolism in the diabetic subgroup (n=125) were both significantly higher in AZA-treated patients (P = 0.008 and 0.045, respectively). Thus, CsA immunosuppression does not appear to be a risk factor for thromboembolic disease, and it may in fact lower the incidence of thromboembolism in diabetic renal allograft recipients.