Thirty patients with renal allografts were monitored for 15 postoperative days by colour-coded Doppler imaging and categorized as stable, rejection, acute tubular necrosis and cyclosporine toxicity. All forms of graft dysfunction showed significantly raised resistance (P less than 0.01) and pulsatility (P less than 0.01) indices (RI and PI, respectively). It was not possible to use Doppler imaging to differentiate the cause of graft dysfunction, but stable grafts in patients with high cyclosporin levels had significantly higher RI (P less than 0.04) and PI (P less than 0.04) than similar grafts in patients with cyclosporine levels in the therapeutic range.