Just prior to performing a renal allograft biopsy with ultrasound guidance, and as part of the routine clinical procedure, duplex measurements are performed by the radiologist who will perform the biopsy. The peak systolic velocity (Vmax) and the minimal diastolic velocity (Vmin) are determined in each patient in two to three representative interlobar arteries. The resistive index is calculated as [1 - (V min/V max)]. This procedure is repeated at three different places in the kidney (lower, interpolar and upper pole). These data are directly entered in the central electronic clinical database system (KWS, see above).