Objective: Peritoneal function during peritoneal dialysis (PD) declines over time due to peritoneal inflammation; however, the immunological mechanism has not been fully clarified. Here, we examined changes in each cellular fraction in the peritoneal dialysis effluent by flow cytometry and their relationship to peritoneal damage.Patients and Methods: We enrolled 23 patients who began PD between 2006 and 2017 and had available datasets of the peritoneal equilibration test and flow cytometric analysis for at least three consecutive visits, with an interval of six months from six months after introducing PD. The levels and changes in each cellular fraction, dialysate/plasma (D/P) creatinine ratio, and the forward scatter (FSC) ratio of mesothelial cells to lymphocytes were compared using a simple linear regression analysis.Results: Among the examined variables, only the fraction of CD8+ TCM cells during the first observation was significantly correlated with the change rate in the D/P creatinine ratio (β=1.47, P=0.001, adjusted R2=0.379). The CD8+ naïve T and CD8+ TCM cell fractions were negatively correlated with the change rate of the D/P creatinine ratio (naïve T cells: β=−0.058, P=0.022, adjusted R2=0.188; TCM cells: β=−0.096, P=0.046, adjusted R2=0.137). In addition, the change rates of the D/P creatinine ratio tended to be higher, though not significantly (one way ANOVA; P=0.080), in accordance with the increase in the change rate of the CD8+ effector memory T cells (TEM).Conclusion: The CD8+ naïve T and TCM cells may transition into TEM cells by repeated exposure to the dialysate over time. The TEM cells residing in the peritoneum may play a significant role in the progression of peritoneal damage.