Reproducible split renal function studies under urea diuresis have made possible the accurate identification of those patients with hypertension secondary to an occlusion of a branch of the main renal artery. Differential renal function studies are presented on 13 patients with segmental renal ischemia due to branch arterial occlusion. These cases all show evidence of excessive sodium and water reabsorption on function study; comparison of the UpAH has been shown to be more diagnostic of renal ischemia than changes in UInulin. The major difficulty in evaluating patients with hypertension occurs in those who exhibit no excessive water reabsorption on differential function study. The important observation for diagnosis in these patients is to compare the ratio of urine flow rates to the ratio of renal mass. In those patients with hypertension as a result of unilateral pyelonephritis renal function studies are of limited value.