In continuous ambulatory peritoneal dialysis (CAPD) residual renal function (RRF) plays an important role in the total amount of weekly clearances of small molecules. The purpose of this study was to determine if there were any differences in certain nutritional parameters between patients with and without RRF, total weekly clearance (KT/V) being equal. Therefore, we compared two groups of patients with equal weekly KT/V: group A without RRF [n = 7, KT/V 2.07 +/- 0.2) and group B with RRF (n = 7, KT/V 2.11 +/- 0.1, urea clearance 1.13 +/- 0.8, creatinine clearance 2.01 +/- 1.5 mL/min, contributing on the average of 15% (range 5.5%-28%) to the determination of KT/V]. The two groups were selected from 52 patients on CAPD for more than 9 months and they were comparable in age (A = 64.6 +/- 7 years, B = 64.1 +/- 7 years), duration of dialysis (A = 39.8 +/- 25 months, B = 36.3 +/- 31 months), body weight (A = 64 +/- 3.9 kg, B = 64.7 +/- 7.4 kg), and body mass index (A = 26.6 +/- 2.9, B = 25.8 +/- 3.6). The two groups turned out to be different in transferrin (A = 209 +/- 51, B = 278 +/- 24 mg/dL, p < 0.006), normalized protein catabolic rate (PCRN) (A = 0.87 +/- 0.07, B = 1.11 +/- 0.07 g/kg/day, p = 0.00), and albumin (A = 3.31 +/- 0.1, B = 3.55 +/- 0.2, p < 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)