The aim of this study was to evaluate longitudinal changes in dialysate cancer antigen 125 (dCA125) levels over time and to analyze relationships between dCA125 and peritoneal glucose exposure (PGE) in children undergoing long-term peritoneal dialysis (PD). The study group included seven boys and four girls (mean age 13 ± 5.1 years) with a mean PD duration of 84.0 ± 1.1 months. A peritoneal equilibration test (PET) was performed, and dCA125 levels were measured in all patients. Peritoneal appearance rates (AR) of dCA125, the velocity of the decrease in dCA125AR values, and annual PGE levels were also calculated. The final tests were performed at a mean of 63.3 ± 3.5 months after the initial ones. Both dCA125 and dCA125AR levels showed statistically significant decrements during the follow-up period (p = 0.003), with the velocity of decrease in dCA125AR found to be 52.6 ± 19.4%. There were no significant differences in peritoneal transport parameters between the beginning and end of the study period. PGE values were significantly higher in the last year of the study than in the first year (p = 0.014), but the velocity of the decrease in dCA125AR levels was not related to total PGE. In conclusion, a significant decline was found in dCA125 and CA125 AR levels, reflecting mesothelial cell mass, in children undergoing long-term PD (>5 years), but these were not related to PGE.