IT IS WELL established that the renal concentrating capacity of normal rats,1,2 dogs,3 and humans (newborns,4 children,5 and adults6) varies directly with the protein content of the diet or the urea available for excretion. Whether this relationship exists when there is an impairment in renal concentrating ability has not been evaluated. A decrease in renal concentrating capacity is virtually a universal feature of sickle cell anemia,7-10 therefore, it was deemed of interest to determine the effect of variation in protein intake on the concentrating ability of children with this disease. Additional data on the concentrating defect in sickle cell anemia is desirable because neither the anatomic nor the physiologic factors responsible for this defect have been defined. Materials and Methods The subjects were nine children with sickle cell anemia and three normal children. Each child received three different protein diets for three-day periods in the following sequence: regular protein (40