Forty-two children aged one to sixteen years with persistent and severe hypertension were investigated by renal vein renin measurements. There were no serious complications in the 49 procedures performed and technical failure occurred on three occasions. Arteriography was performed in 35. Asymmetrical renin release was found in 22 patients and of these 15 underwent surgery. This was successful in 12 patients (80%) who became normotensive. Ten had unilateral disease (100% cure rate) but only 2 (40%) with bilateral disease became normotensive. Renal vein renin studies combined with arteriography have a useful role in the investigation and management of childhood hypertension.