The gain in the diagnostic power of the routine preoperative bilateral renal vein renin investigation by the inclusion of an acute stimulation of renin release was studied in 25 hypertensive patients with fibromuscular (FMD) and 44 with arteriosclerotic (AS) renovascular disease. Nine FMD and 17 AS patients had renal vein renin ratios greater than or equal to 1.50 under unstimulated conditions. Of these 26 patients, 8 in each group improved after surgical treatment. Among the 14 FMD and 24 AS patients with ratios less than 1.50, another 9 FMD and 7 AS patients improved after operation. After acute stimulation of renin release, no less than 18 FMD and 26 AS patients had a unilateral renin secretion from the diseased kidney and of these, 14 FMD and 14 AS patients were successfully treated with operation of the stenosis. Thus the stimulation was necessary for correct preoperative diagnosis in 38% of 32 successfully operated patients, and at the same time the predictive value of a negative test increased from 0.58 to 0.80. It is concluded that acute stimulation of renin release greatly improves the diagnostic power of the renal vein renin investigation in renovascular hypertension.