In recent years measurements of arterial blood flow in the renal allograft have been studied in an attempt to provide a non-invasive method of diagnosing and monitoring post-transplant renal dysfunction. Many studies have involved measurement of the resistance index (RI) of arterial blood flow. Although the reproducibility of the technique has been studied in other areas of application, no such studies have been performed on the renal allograft. In this study we attempted to evaluate the reproducibility of the measurement of the RI of the interlobar artery in the renal allograft. 18 renal allograft recipients with stable functioning grafts were studied twice by each of two experienced sonographers using a colour-coded duplex Doppler ultrasound scanner, in order to determine the intraobserver and interobserver variability in the measurement of RI of the interlobar arteries in the allograft. Variability was small and although interobserver variability exceeded intraobserver variability, both were within repeatability limits adopted by the British Standards Institution. The results suggest that measurement of the RI of the interlobar artery of renal allografts is repeatable and reproducible when performed by experienced operators.