For 15 years, perfusion indices derived from scintigraphic studies have proved useful in the serial evaluation of renal transplants and they have recently been confirmed as being more sensitive than Doppler ultrasound as an indicator of vascular rejection and cyclosporin toxicity. In the calculation of these indices, correction for administered activity is often accomplished using activity measurements made over a convenient artery which, therefore, has a critical influence on the value of the index obtained. In this communication, a theoretical assessment is made of the error and variability introduced into the calculation of the perfusion index, because of inadequate spatial sampling of activity in these narrow arteries and the consequential inconsistencies in the measurement of the arterial tracer activity. Using numerical simulation, it is shown that the errors in repeat studies on the same patient may be as high as 39% and between patients as high as 53%. These figures can be reduced to below 18% and 21%, respectively, by constructing a region of interest (ROI) to extend over as much of the arterial width as possible rather than relying only on the maximum pixel count. Further reduction to below 12% and 10% is possible by utilising a 128 x 128 acquisition matrix instead of 64 x 64 and drawing the ROI over the aorta instead of the iliac artery.