The ability of the kidneys to concentrate urine is regarded as a sensitive indicator of renal function.1Â Since concentration tests are easily done, they enjoy wide clinical use. This renal activity is usually studied by measuring urine specific gravity, i.e. the ratio of the weight of urine to an equal volume of water. The specific gravity of a solution is dependent upon the weight of the particles making up the solution. However, specific gravity will afford only a rough index of renal concentrating ability since the kidney, in the maintenance of water and electrolyte balance, responds to changes in the osmolality of body fluids, not the specific gravity.2Â This information is only slowly penetrating clinical medicine.There have been some studies relating maximum urine osmolality to the hypertensive state, diabetes mellitus, rheumatic heart disease,2,3Â and sicklemia.4Some relationships between urine osmolality and urine specific gravity