A female patient with a side-to-side fistula between the radial artery and the cephalic vein complained of pain in the three radial fingers during dialyses. Using 133-Xenon and histamine injected locally into the thumb musculature, local effective arterial pressure was found to be decreased, and to rise after compression of the fistula, or the radial artery distal to the fistula. The patient's symptoms thus were probably ischaemic and disappeared after ligation of the radial artery distal to the fistula. Ten more patients, of whom 9 had no local symptoms, were studied using the same technique. In 3, systolic blood pressure was measured in the thumb by a strain gauge technique. Some "steal" of thumb blood pressure by the fistula could be demonstrated in all patients but one and retrograde flow of blood in the artery distal to the fistula in 8. "Supernormal" Xenon wash-out occured when the pressure head was restored to normal by compression of the fistula or the radial artery distal to it. This is probably due to a compensatory increase of the distal vascular bed. The technique can be used for investigation of patients with suspected ischaemia distal to an arteriovenous fistula, and to indicate the appropriate treatment, usually ligation of the artery distal to the fistula.