Kinetics of phosphate removal, based on hourly collection of used dialysate or filtrate and hourly changes of phosphate plasma concentration, were studied in hemodialysis (QB 300; QD 500 ml/min), hemodiafiltration (QB 300; QD 500; QSF 25 ml/min), and hemofiltration (QB 250; QSF 70 ml/min) for six 5-hour sessions in each mode of therapy. Whatever the pretreatment phosphate concentration (1.5-2.0 mmol/L range), and whatever the treatment modality used, final plasma phosphate concentration was in the narrow range of 0.8-0.9 mmol/L, and about 50% of the total mass transfer occurred during the first 2 hours. At the third hour, a steady state is reached, suggesting that removal of phosphate is limited by the rate of phosphate transfer from body compartments to extracellular fluid, which was on the average about 362 mumol/kg.hr. Consequently, total phosphate mass transfer accounts only for 20 to 28 mmol per session. Control of pretreatment phosphatemia in the range of 1.5 to 2.0 mmol/L depends on daily phosphate binder prescription, calcitriol supplementation, and control of metabolic acidosis; one cannot rely on intermittent phosphate removal during the dialysis session.