This prospective study was conducted to assess dialysate cancer antigen 125 (dCA125) levels in pediatric peritoneal dialysis (PD) patients and to investigate whether it exhibits any alterations during or after recovery from peritonitis, and also to analyze the relationships between dCA125 and age, duration of PD treatment, peritonitis incidence, time passed since the last episode of peritonitis, PD prescription parameters, and peritoneal transport parameters. Forty-seven standardized 4-h peritoneal equilibration tests (PET) were performed in 38 children (mean age 11.6+/-4.7 years) on PD (duration 20.9+/-14.3 months). Thirty-two of the patients were in stable condition at the time of PET (stable group). Six patients were included in the study only during a peritonitis episode, and two of the stable patients were reevaluated during a peritonitis episode afterwards (peritonitis group). Seven out of a total of eight patients with acute peritonitis were reexamined after recovery (recovery group). CA125 levels were measured in 188 samples at 0-, 1-, 2-, and 4-h dwells. Peritoneal appearance rates (AR) were calculated. Mean dCA125 (4 h) and AR CA125 values were 5.6+/-5.3 U/ml [median 4.15 U/ml/4 h (range 0.5-25.9)] and 50.1+/-45.6 U/min/1.73 m2[median 37.91 U/ml/1.73 m2 (range 3.61-223.39)]. AR CA125 levels did not correlate with age, PD duration, peritonitis incidence, time passed since the last peritonitis episode, exchange volume used per m2 per day, or peritoneal transport properties in the stable patients' group. Although stable patients using hypertonic PD solutions (n=16) had slightly lower AR CA125 levels (p=0.04), multivariate analysis showed no influence of hypertonic dextrose solutions on mesothelial CA125 secretion (p=0.4). During acute peritonitis, CA125 concentrations showed a reversible threefold increase [AR CA125: stable 37.9 vs. peritonitis 101.2 U/ml/1.73 m2 (p=0.001)]. No difference could be found between the stable group and the recovery group. We conclude that changes in the peritoneal mesothelial cell mass cannot be assessed by determining CA125 in a cross-sectional way and that longitudinal determinations could be more valuable in the follow-up of patients.