The renal veins are paired blood vessels that carry filtered blood from each kidney back to the inferior vena cava. The left renal vein has a longer course, passing between the aorta and the superior mesenteric artery (a space that can cause compression known as “Nutcracker syndrome”). On ultrasound, the renal veins are evaluated for patency and flow. Renal vein thrombosis — a clot within the renal vein — can be seen as echogenic material filling the vein with absent or decreased Doppler flow. This condition may be associated with nephrotic syndrome, renal cell carcinoma (which can extend tumor thrombus into the renal vein), or dehydration in infants. Assessment of renal vein flow is also important in transplant kidney evaluation and in patients with suspected Nutcracker syndrome.