Preoperative and postoperative plasma renin activity (PRA) was determined serially in 23 patients undergoing operation for renovascular hypertension. In 18 patients PRA became normal within five days postoperatively and preceded normalization of the blood pressure. Two patients, with elevated PRA postoperatively, required secondary nephrectomy, following which PRA and blood pressure became normal. These 20 patients have been observed for an average of 29 months; 15 have had an excellent result (diastolic blood pressure of 90 mm Hg or less) and five a good result (blood pressure as above, but requiring drugs). In contrast, the three patients with persistent elevation of postoperative PRA had refractory hypertension and one hypertensive death occurred. Measurement of PRA following operation for renovascular hypertension provides a more direct and immediate assessment of operative success or failure than determination of blood pressure alone and allows a prediction of long-term results.