Elevated serum phosphorus levels are associated with increased morbidity and mortality in dialysis patients. To study the determinants of serum phosphorus levels and phosphorus clearance, we measured phosphorus concentrations in 24-hour collections of dialysate and residual urine output during routine urea kinetics in 56 peritoneal dialysis patients. Dietary records were used to estimate oral phosphorus intake, and the net gastrointestinal absorption of phosphorus was determined measuring the amount of phosphorus excreted in 24 hours. Dialysate to plasma ratios of phosphorus and creatinine were very similar, and the peritoneal clearances of phosphorus and creatinine correlated well and were of the same magnitude. High transporters in the peritoneal equilibration test had greater phosphorus and creatinine clearances than low transporters. In patients with residual urine output, the renal clearances of phosphorus and creatinine also correlated well. Comparing two groups of patients with a similar Kt/V of 2 and greater but different levels of creatinine clearance, either greater or less than 60 L/wk, the patients with the greater creatinine clearance also had a greater phosphorus clearance (7.0 ± 2.2 versus 4.3 ± 0.7 mL/min/1.73 m²) and significantly lower serum phosphorus levels (4.8 ± 0.9 versus 5.9 ± 1.4 mg/dL) despite a greater daily amount of phosphorus absorption (490 ± 190 versus 336 ± 100 mg/d). In summary, creatinine clearance measurements provide a good estimate of phosphorus clearance and how much dietary phosphorus and therefore protein can be tolerated.