The importance of immortal time in observational studies that have time-dependent treatment effects has been re-emphasized recently.1 The widespread use of advanced (physiologic pH, biocompatible bicarbonate/lactate-buffered) peritoneal dialysis solution in Korea has permitted the development of observational data sets that have been used to study the impact of these solutions on patient mortality. In our previous study, which used a data set of 2,163 patients enrolled from 54 Korean centers, we observed a significant association between improved patient survival and use of an advanced peritoneal dialysis solution.