Surgically constructed Brescia-Cimino arteriovenous fistulas and synthetic loop grafts are common means of vascular access for hemodialysis. Although angiography has been the traditional method of imaging these vascular systems, duplex and color Doppler sonography offer a noninvasive method of evaluating dysfunctional hemodialysis access. In normally functioning fistulas, waveforms of flow in the supply arteries and throughout the graft are monophasic, with peak systolic velocities of 100-400 cm/sec and end-diastolic velocities of 60-200 cm/sec. The draining veins have arterial pulsations with peak velocities of 30-100 cm/sec. Arterial and venous stenoses, graft thrombosis (occlusive and nonocclusive), infection, aneurysm and pseudoaneurysm formation, and arterial steal are relatively common abnormalities that can threaten or destroy graft function and can be diagnosed sonographically. Although abnormal hemodynamics in access fistulas are usually detected during hemodialysis, sonographic evaluation at the time of initial dysfunction may reveal an underlying correctable abnormality, and specific therapy may be instituted before the condition progresses. In addition, use of sonography may obviate an invasive angiographic examination if no significant hemodynamic problem is present.