We collected data on 43 stable peritoneal dialysis patients (60 values), all of whom had residual renal function, to determine the dependency of normalized protein catabolic rate (PCRN) on residual renal function as well as dialysis adequacy. All patients were on a stable peritoneal dialysis regimen for at least 4 months (median 8 months, range 4-64 months) and were without peritonitis or other serious infections for at least 6 months prior to data collection. Dialysate creatinine clearance (Ccr) was obtained from 24-hour collections of dialysate. Residual renal function was calculated from the average of the urea and the creatinine clearance from 24-hour urine collections. The normalized protein catabolic rate was calculated using the Randerson method. Using linear regression analysis, PCRN correlated with total weekly Ccr (r = 0.7, p < 0.0005), KT/V urea (r = 0.5, p < 0.0005), and residual renal function (r = 0.6, p < 0.0005). All patients with residual renal function more than 40 L/week had a PCRN greater than 1.0 g/kg/day. To assure a PCRN of 1.0 g/kg/day in all patients, a weekly KT/V urea of 1.9 and/or a total weekly Ccr of 79 L was required. These results confirm the important role of residual renal function in dialysis adequacy and elucidates its contribution to the maintenance of the patient's protein intake.