Of 37 patients who underwent successful renal revascularization for unilateral renovascular hypertension, 27 were cured and ten improved. The two most reliable selective criteria for operation were a renin ratio of 1.40 or greater and arteriographic demonstration of collateral channels circumventing the stenotic lesion. Twenty-five patients (68%) had collateral vessels documented arteriographically; 12 did not. Patients lacking collaterals had renin ratios of at least 1.4 with a mean ratio of 2.5. Individuals with collaterals had a mean ratio of 1.8. This suggests that as collaterals develop, ratios revert toward unity. Thirty patients had ratios of 1.4 or greater. Collaterals, indicating a hemodynamically significant lesion, were demonstrated in all seven patients who had ratios under 1.4. These seven patients would have been denied surgical benefit had renin ratios been the only selective criteria.