A diagnosis of renal vein thrombosis in childhood may be reliably confirmed by inferior vena cavagraphy, using conventional roentgenographic techniques with urographic contrast media. Bilateral renal vein thrombosis and with this, thrombosis of the inferior vena cava, are more common than has been recognized; radionuclide vena cavagraphy can demonstrate this and is conveniently performed by injecting the radionuclide bolus via a leg vein with rapid sequential imaging. The vena caval obstruction, being at the level of the renal veins, does not lead to any apparent disturbance of venous drainage from the lower limbs. Renal vein occlusion is followed by a variable degree of renal parenchymal damage. The end result may be a normal kidney or a completely necrotic one. Studies of renal perfusion, by either transumbilical aortography in the neonate or radionuclide angiography in the older child, will provide an estimate of the prognosis in an affected kidney. Hypertension is an occasional sequel and may be cured by nephrectomy.