To evaluate the effects of metabolic acidosis on serum albumin and other nutritional parameters in long-term continuous ambulatory peritoneal dialysis (CAPD) patients, we undertook a retrospective study involving 106 CAPD patients who had monthly biochemical measurements and urea kinetic studies every 6 months for more than 2 years. The patients were divided into three groups according to their mean total CO2 (tCO2) level of the 2-year follow-up (Group I: mean tCO2 < 22 mmol/L; Group II: 22 mmol/L < or = tCO2 < 26 mmol/L; Group III: mean tCO2 > or = 26 mmol/L), and the clinical, biochemical, and urea kinetic data were compared between the three groups. The mean tCO2 in Groups I, II, and III were 20.62 +/- 1.2 mmol/L, 23.91 +/- 1.1 mmol/L, and 27.3 +/- 0.8 mmol/L, respectively. The percentage of body weight (Bwt) to ideal body weight (IBW) was significantly higher in Group I (113.1 +/- 15.3%) compared to Group II (103.5 +/- 11.5%) and Group III (98.7 +/- 8.0%) (p < 0.05), but the percentage of lean body mass (LBM) to Bwt was not different between the three groups. Compared to Group III, Group I had significantly higher blood urea nitrogen (BUN) (61.1 +/- 14.3 vs 46.1 +/- 7.2 mg/dL, p < 0.05), serum albumin (4.04 +/- 0.31 vs 3.75 +/- 0.39 g/dL, p < 0.05), and normalized protein equivalent to nitrogen appearance (NPNA) (1.02 +/- 0.21 vs 0.88 +/- 0.14 g/kg/day, p < 0.05), and more ultrafiltration volume (1.4 +/- 0.4 vs 1.0 +/- 0.3 L/day, p < 0.05), in spite of comparable dialysis dose and albumin loss into the dialysate. No differences were observed in the three groups in the changes of tCO2, Bwt/IBW, LBM/Bwt, BUN, and albumin from the baseline values after the 2-year follow-up. Using stepwise multiple regression analysis, NPNA, Bwt/IBW, and ultrafiltration volume were independent factors affecting mean tCO2 level. In conclusion, low tCO2 levels in long-term CAPD patients may reflect increased protein intake, and the mild to moderate degrees of metabolic acidosis may not affect the nutritional status of well-dialyzed CAPD patients.