A case of medullary cystic disease and chronic renal failure with severe sodium wastage is described. Urinary sodium and calcium concentrations were fixed at a level approximating plasma ultrafiltrate. Renal salt loss was flow dependent. It is suggested that the main cause of the severe salt loss was a tubular and medullary defect of sodium transport; hyperfiltration and increased osmotic load per nephron played a contributory role.In the patient described, calcium was excreted in the urine in quantities that were unusually large for chronic renal failure but were commensurate with the renal salt loss.