A pulsating mass with an audible thrill was present on the left foreleg of a dog for approximately one year duration. Auscultation of the mass revealed a machinery-type murmur. Based on the clinical signs, the leading differential diagnosis was an arteriovenous (AV) fistula. A computed tomography scan, an angiogram, and a reconstructed 3-D CT scan confirmed the presence of a fistula. An initial attempt was made to ligate the main feeder artery to the fistula, which resulted in a 50% decrease in the size of the mass and cessation of pulsation. The mass returned 5 months later. Color Doppler ultrasound and angiography revealed an anastomosing network of vessels with a centrally located aneurysm that was unable to be occluded digitally. The fistula was excised after dissection and ligation of feeder and collateral vessels surrounding the aneurysm. Prognosis for AV fistulas is guarded. Complications of untreated or unsuccessfully treated AV fistulas include increased sympathetic stimulation to the heart due to decreased organ perfusion leading to increased cardiac output and congestive heart failure. Amputation of AV fistulas of extremities is the most reliable way to prevent cardiac failure and ensure a better prognosis.