In transplanted patients graft rejection is the most frequent complication in the first year after surgery. Vascular lesions (necrosis, intimal proliferation, thrombosis) are signs of poor prognosis and lead to irreversible loss of renal function and graft removal in most cases. The problem of vascular rejection is still not solved and the results of therapy unsatisfactory, both because of inadequacy of diagnosis and/or inadequacy of available therapy. The role of prevention, very likely the best approach, is still sub judice. In an attempt to explore the validity of prevention, 22 transplanted patients (group A) were given a new antithrombotic agent (defibrotide) immediately after surgery, and the results were compared with those of a well-matched group of 30 patients on dipyridamole (group B). Follow-up lasted 6–19 months (mean 9.9) for group A; 5–21 months (mean 12) for group B. In group A, 8 patients (36%) had rejection episodes. Antirejection therapy was followed by recovery of renal function in all cases. In group B, 9 patients (29%) had rejection crises and graft removal was necessary in 7 instances due to severe vascular lesions. At the end of follow-up, all patients treated with defibrotide had normally functioning grafts: among the 30 patients on dipyridamole, 22 (73%) had satisfactory graft function.