In this pilot study, sequential ultrasonic scanning was used to assess (a) whether furosemide injection normally causes an enlargement of kidney size, and (b) whether this effect is blunted in renovascular hypertensives to such a degree as to bear potential diagnostic value. Furosemide caused similar degrees of maximal area enlargement in both kidneys of 10 healthy subjects (16.1 +/- 3.9%) and 10 essential hypertensives (14.8 +/- 4.2%) whereas in 11 renovascular patients maximal distension of the affected kidneys was significantly blunted (9.4 +/- 3.7%) in comparison with that of contralateral kidneys (19.6 +/- 7.7%). In the 6 hypertensives with one small kidney not due to renal artery stenosis, maximal enlargement of the affected kidneys averaged 22.8 +/- 5.1%, and that of contralateral kidneys 15.8 +/- 4.2%. In 8 out of the 11 patients with renovascular hypertension the difference in response between the paired kidneys had values outside the ranges of patients with both the other forms of arterial hypertension. The data suggest that the furosemide test may be of value in the screening for renovascular hypertension.