We analyzed renal sodium handling in a patient with Bartter's syndrome after equilibration on diets containing 200 and 700 mmol NaCl/day. No potassium supplements were given. Clearance studies were performed during maximal water diuresis, maximal free water clearance was taken as measure of diluting segment sodium reabsorption. A normal reference frame of diluting segment NaCl reabsorption at varying delivery rates to that segment was drawn from data collected in similarly studied normal subjects after equilibration on low (20 and 200 mmol/day) or very high (916-1,224 mmol/day) sodium intakes. The results were compatible with a defect in diluting segment reabsorption in the patient, which appeared more pronounced during the high sodium intake. Recent reports do not agree with the existence of such a defect in Bartter's syndrome. One of the reasons of this difference may be the lack of a proper reference frame in previously published studies. We, therefore, compared clearance data in adults with Bartter's syndrome published by others with our studies in normals. Our analysis showed that in Bartter's syndrome (1) a moderate diluting segment NaCl reabsorption defect is often present, and (2) the proximal reabsorption is frequently elevated relative to sodium intake. Severely suppressed diluting segment reabsorption and relatively low proximal reabsorption may also occur, particularly in association with salt loading or reduced glomerular filtration. It seems, therefore, that much of the continuing controversy on renal sodium handling in Bartter's syndrome may be solved by using the proper frame of reference.