Color Doppler imaging was used to identify and localize arteriovenous fistulas in six patients after cardiac catheterization. In all six, there was a focal region of persistent intraarterial flow throughout the cardiac cycle at the site of the fistula, in contrast to normal arterial flow, which is forward only in systole. In all cases, turbulent pulsatile venous flow was present at the site of the fistula and nonturbulent pulsatile venous flow was present proximal to the fistula. In five cases a focal area of apparent extravascular tissue vibration was identified. In four cases the communication between the artery and vein was identified. Familiarity with these color Doppler imaging findings should allow for accurate noninvasive diagnosis of arteriovenous fistulas, at the patient's bedside if necessary.