Five patients with sonographic evidence of a dilated renal vein are described. The etiology of the dilated renal vein was tumor thrombus in two patients, neoplastic arteriovenous shunting in two, and portal systemic shunting into the left renal vein in one. Tumor thrombus had echogenic enlargement of the renal vein, whereas anechoic enlargement of the renal vein was secondary to increased blood flow within the renal vein. Portorenal shunting can be differentiated from a neoplastic renal arteriovenous fistula by the presence of thrombus within the portal system and the absence of a renal mass. Pitfalls in the diagnosis of renal vein enlargement are discussed.