Many renal structural and functional abnormalities have been associated with sickle cell disease. The patients have an impaired urinary concentrating ability but an intact diluting capacity. There are defects in both urinary acidification and potassium excretion, although overt metabolic acidosis and hyperkalemia occur infrequently. Proximal tubular function is supranormal, as manifested by increased reabsorption of phosphate and increased secretion of creatinine. The former results in mild hyperphosphatemia, while the latter causes substantial overestimation of the glomerular filtration rate (GFR) by creatinine clearance. Both GFR and renal plasma flow are increased in young patients with sickle cell disease, but prostaglandin inhibitors decrease the GFR. The GFR progressively decreases with increasing age. Proteinuria, and even nephrotic syndrome, are relatively frequent; the most common renal lesion in children is focal glomerular sclerosis, which may be associated with progressive deterioration in renal function. Glomerular hyperfiltration has been implicated in the pathogenesis of the glomerular lesions, as well as in the development of renal failure. In patients with end-stage renal disease, both hemodialysis and kidney transplantation have been successful. Recurrent hematuria is a relatively common problem in patients with sickle cell disease. The bleeding usually remits spontaneously, but occasionally requires therapy with aminocaproic acid. Papillary necrosis may occur, and is thought to result from medullary ischemia.