Congenital arteriovenous renal fistulas are rare malformations due to abnormal communications between arterial and the venous systems. There are two types of congenital arteriovenous malformations: crisoid or, as in the present study, aneurysmal. Hematuria is the major and most common symptom, along with other clinical manifestations, such as hypertension, left ventricular hypertrophy, cardiac failure, and abdominal pain, but the congenital aneurysmatic arteriovenous renal fistulas can be also asymptomatic. Diagnosis can arise from a focused survey, suggested by a medical case or to be occasional, as in the present case of study. Ultrasonography with color duplex studies is the first line of imaging studies used in the diagnosis of renal arteriovenous malformations, The differential diagnosis must be made with other anechoic lesion: abscesses, tumors, hydronephrosis or, as in this case, a renal cyst. Angiography is the gold standard in the diagnosis of arteriovenous malformations, especially in those cases where the diagnostic -therapeutic treatment requires the endovascular treatment of the vessel, as in the case of a 46 years old man submitted in our clinic to the ultrasonography follow-up for a renal cyst.