Su Jin Kim and coworkers from Korea published an important study on the relationship of residual renal function (RRF) and cystatin in pediatric peritoneal dialysis (PD) patients in this issue of Pediatric Nephrology, both in anuric patients and patients with RRF. Based on a lack of correlation between cystatin C and standard small solute-based dialysis adequacy parameters such as Kt/Vurea but a significant correlation with RRF, the authors concluded that cystatin C may be a good tool to monitor RRF. The editorial reviews the available literature in adults, the different handing between urea and cystatin C, and the determinants of cystatin C clearance in dialysis patients. In adults, cystatin C levels are determined predominantly by RRF, but not exclusively. In anephric hemodialysis and PD patients, there is a correlation with standard weekly Kt/Vurea. Cystatin C levels will also depend on ultrafiltration. Despite these factors that affect cystatin C levels beyond RRF, cystatin C is a useful parameter for monitoring PD patients that may be more closely related to long-term outcomes than small solute adequacy parameters.