A dye density test for diagnosis of renovascular hypertension is described. Dye densities are measured by the Goldberg TC refractometer and the Hitachi hand refractometer. The test is easily done. Curves of urine densities are then drawn, representing differential function in terms of renal tubular concentrating ability. Hyperconcentration of the dye occurred consistently with positive Howard, Rapoport and Stamey tests. The dye density test places emphasis on the concentration of the urographic media rather than on the urinary volume. The most satisfactory attribute of the dye density test has been its constant reliability with the smaller ureteral catheters, which have greater advantages of minimizing adverse side effects reported by others using larger catheters. There may be a significant place for the dye density test in the routine evaluation of hypertension, because the test is carried out with ease and without adverse side effects, the results are immediately available, urine densities can be measured by the local hospital laboratory, and there is a consistent correlation among dye density measurements, Howard, Rapoport and Stamey tests.