Residual renal function (RRF) is an important parameter in the management of patients on chronic dialysis. The aim of this cross-sectional study was to determine the efficacy of serum cystatin C (CysC) for RRF estimation in 20 children (16 boys, 4 girls; median age 13.4 years) undergoing peritoneal dialysis (PD). For studies of correlation with serum CysC, the average of creatinine clearance rate (Ccr) and urea clearance rate (Curea), Kt/Vurea, and weekly Ccr were evaluated as parameters reflecting RRF. The serum CysC level was found to be negatively correlated with urine volume (r=-0.717, P<0.001), average of Ccr and Curea (r=-0.851, P<0.001), total and renal weekly Ccr (r=-0.795, P<0.001; r=-0.845, P<0.001, respectively), and renal Kt/Vurea (r=-0.793, P<0.001) and positively correlated with peritoneal weekly Ccr (r=0.738, P<0.001) and peritoneal Kt/Vurea (r=0.785, P<0.001). There was no significant association with total Kt/Vurea (r=-0.335, P=0.148). In non-anuric group of patients, serum CysC had no link to peritoneal Kt/Vurea (r=0.573, P=0.066), but was negatively correlated with renal Kt/Vurea (r=-0.609, P=0.047). In the multiple regression analysis, renal Kt/Vurea significantly contributed to log CysC concentration rather than peritoneal Kt/Vurea. The results of this study suggest that serum CysC could be an appropriate marker for RRF, independent of total and peritoneal Kt/Vurea.