Cyclosporin (CyA) associated renal dysfunction may be due to an arteriolopathy which reduces graft perfusion. We have prospectively monitored 27 renal allograft recipients, within 30 days of transplantation, by continuous wave Doppler ultrasound. Changes in the frequency shift waveform should indicate alterations in renal blood flow. Objective analysis was made in 112 waveforms by determining the ratio of the area under the frequency shift/time curve of the first half and the second half of the cardiac cycle (AUC 1/2 ratio). The AUC of the first half of the cycle indicates systolic blood flow plus the first part of diastolic flow, whereas the AUC of the second half indicates the remainder of diastolic flow. The AUC 1/2 ratio during ATN was not significantly different from that during stable renal function. However the ratio was significantly higher during acute rejection episodes and CyA nephrotoxicity. Although a statistical difference was demonstrated between acute rejection and CyA nephrotoxicity, the AUC 1/2 ratio alone does not differentiate these 2 conditions. These preliminary findings suggest CyA nephrotoxicity does reduce renal allograft blood flow and that this non-invasive technique may have a useful role to play in monitoring of patients immunosuppressed with CyA.