In order to evaluate the ultrasonographic measurement of kidney length, 32 hypertensive patients with renal arterial stenosis treated by intraluminal angioplasty were examined by several ultrasonographies performed by the same observer before and after dilatation. The lack of variation in the contralateral kidney length ascertained the intra-observer reproducibility of the method. We found that ischaemic kidneys were smaller and that this diminution in size depended on the renal arterial lesion (kidneys below atheromatous lesions were of smaller size than those below fibrodysplastic lesions). The increase in size of kidneys treated by angioplasty was most probably due to an increase in perfusion pressure and was to be compared with the results of a separate evaluation of renal function. However, this method has no individual applications in view of the important kidney length distribution at cross-checkings.