Renal-vein renin was determined in 55 children with renovascular or renal hypertension, 33 of whom being treated surgically. The overall cure rate after surgery was 87.9%. Of 23 patients having a renal-vein renin ratio greater than 1.5, 20 were operated successfully, but 9 of 10 operated patients with a ratio of less than 1.5 also benefited from surgery. Taking into account the frequency of "false-negative ratios" (27.3%) and the potential risks of catheterization, it is suggested that renal-vein renin measurements are not useful as a routine procedure for investigation of a child with renovascular hypertension.