Renal vascular changes were seen in six patients after renal homotransplantation. A deposition of fibrinous material on the intimal surface and in the vessel wall was found in three cadaver kidneys where extensive areas of necrosis had occurred. Necrotizing arterial lesions were seen in two transplanted kidneys and may have represented acute rejection. Severe obliteration of the vessel walls in four patients was considered to be the end result of recurrent acute rejections or of chronic rejection. It is hoped that vigorous immunosuppressive therapy may prevent vascular rejection.