Congenital renal arteriovenous fistula (cirsoid aneurysm).

"Cirsoid aneurysm" represents the pathologic substratum of congenital arteriovenous fistula. Congenital renal arteriovenous fistulas are rare clinicopathologic entities usually first manifested by hematuria. Recently, several studies of this malformation have been reported, but few have made reference to the pathologic findings. We report 2 cases of this lesion made up of arteries, veins, and nondeterminated vessels localized between calyceal or pelvic wall and renal parenchyma. In addition, both cases showed signs of both old and recent hemorrhages, explaining the upper tract bleeding.