In a prospective analysis we performed 182 colour duplex US studies in 60 patients with renal transplants to determine the effects of different causes of graft dysfunction on the resistive index (RI) and the colour imaging pattern of the transplant arteries. Allografts with normal function, cyclosporin toxicity, cytomegaly infection, acute tubular necrosis, glomerulonephritis and postoperative functional nephrotoxicity showed normal RI (less than 0.7) and normal pattern of the arteries in colour duplex. Grafts with acute and chronic rejection, obstructive uropathy and arteriolosclerosis showed significant elevated RI and in colour duplex always a characteristic blinking of the arteries and a numerical reduction of the peripheral arteries. It seems that the assessment of an increase of vascular impedance is possible also with colour duplex US.