Left renal vein entrapment was documented by bilateral ureteral catheterization and imaging studies as a cause of orthostatic proteinuria in two girls. Renal ultrasonography showed compression of the left renal vein between the aorta and the superior mesenteric artery (Nutcracker phenomenon). Abnormal collateral veins and high pressure gradients between the left renal vein and the inferior vena cava were found on left renal venography and pressure tracing, respectively. The left kidney was documented as the source of postural proteinuria by bilateral ureteral catheterization. Our observations suggest renal congestion due to left renal vein entrapment was the cause of orthostatic proteinuria.