Attention has been focused on cancer antigen 125 (CA125) levels in dialysate as an index for evaluating the severity of peritoneal mesothelial cell impairment. However, the method of evaluating CA125 levels in the dialysate has not been standardized. We determined that CA125 levels in the dialysate should be corrected by body surface area (BSA). Using 51 patients who had been undergoing peritoneal dialysis (PD), CA125 levels in the dialysate were measured to evaluate the appearance rate of CA125 (CA125AR). Because only 18 of the 51 patients (35%) had a BSA greater than 1.73 m2, CA125 levels in the dialysate might be underestimated in other patients when absolute values of CA125 are not corrected by BSA. When correlations between corrected CA125AR and various parameters were evaluated, corrected CA125AR positively correlated with age, but negatively correlated with serum creatinine and albumin levels. Moreover, no positive correlation was seen between corrected CA125AR and the duration of PD or the peritoneal permeability test. However, in 5 patients with ultrafiltration loss after prolonged PD, the values of CA125AR persistently or rapidly decreased before transfer to hemodialysis. Although CA125 can be used as an index of peritoneal deterioration, the absolute value of CA125 should be corrected by BSA. However, it is most important to evaluate changes in CA125 in individual patients.