♦ Background and Objectives: The extent to which hemo globin (Hb) cycling occurs in peritoneal dialysis (PD) patients is unclear. It is also uncertain whether different types of erythropoiesisstimulating agents (ESAs) affect such cycling. We performed a retrospective cohort study of our PD population before and after the entire program was switched from epoetin beta (NeoRecormon: HoffmanLaRoche, Basel, Switzerland) to continuous erythropoietin receptor activator [CERA (Mircera: Hoffman-LaRoche)]. ♦ Design, Setting, Participants, and Measurements: The study included 79 patients receiving PD for endstage renal failure and being treated with an ESA. Hemoglobin concentrations were measured monthly, and each study period ran for 12 months. Patient demographics and details of intercurrent illness and hospital admission were collected. ♦ Results: There was a trend to fewer patients on CERA (26 patients, 68.4%) than on epoetin beta (36 patients, 87.8%, p = 0.054) experiencing Hb excursions. The CERA group also required fewer dose changes. However, there was no differ ence in the proportion of patients experiencing complete Hb cycles. On logistic regression, the factors associated with Hb cycling were ESA dose increase or decrease and hospital admission. We also observed a positive correla tion between the delta ESA dose and the amplitude of Hb excursion, suggesting that the dose changes were causal, rather than reactive. ♦ Conclusions: Hemoglobin cycling occurs in PD patients and is largely a consequence of current practice in ESA dos ing, plus the effects of intercurrent illness. The longer half life of CERA may offer a small advantage in reducing the degree of Hb variability, possibly because of fewer dose changes per patient.