The reliability of the radioisotope renogram in detecting functional disparity in the separate kidneys of hypertensive patients with unilateral renal disease was investigated. Renograms recorded in 33 hypertensive patients without unilateral renal disease were employed for establishing control criteria. The physiologic basis for the three types of positive renograms found in the presence of unilateral renal dysfunction has been discussed. The renogram appears to be a valuable method for demonstrating significant disparity in separate kidney function while avoiding the hazards and technical difficulties of bilateral ureteral catheterization.