Thirty episodes of histologically verified acute vascular rejection in kidney transplant recipients were studied. In 11 grafts the rejection was mainly vascular, whereas in 19 grafts a concomitant cellular rejection was seen. Histological features prognostic for bad outcome were glomerular necrosis and thrombi in the arteries and arterioles. Characteristic findings in transplant cytology, i.e., high number of monocytes and low number of lymphocytes and blast cells were noted prior to the onset of clinical signs of rejection, and this finding was also persisting throughout the rejection episode. The numbers of lymphocytes and blast cells were significantly lower in grafts with a pure vascular rejection than in grafts with a concomitant cellular rejection. Vascular rejection was reversible in 15 cases. As rescue therapy plasmapheresis and added immunosuppression were often successful.