Renal DTPA studies were analysed to produce numerical data of renal function (blood flow, glomerular filtration, and excretion), and this was used as an adjunct to the routine imaging information in a study of renal artery stenosis (RAS). The results show an overall accuracy of 81%, with a sensitivity of 96% and a specificity of 61%. In patients with RAS, beta-blocking drugs reduced the difference between the two kidneys. ACE-inhibiting drugs appeared to preserve renal blood flow but also to cause a deterioration in the glomerular filtration rate of kidneys with RAS. An explanation is proposed, in which renal capillary pressure is more important for function than is renal blood flow.