OBJECTIVES: To compare the within-patient variability in haemoglobin levels in pre-dialysis or peritoneal dialysis patients receiving intravenous epoetin alfa or intravenous darbepoetin alfa. METHODS: Data on pre-dialysis (n=169) and peritoneal dialysis (n=162) patients were extracted from the Renal Anaemia Management (RAM) database from 2003 to 2004. The variance in haemoglobin was calculated for each patient with more than one haemoglobin observation. A mixed-model was fitted to the within-patient variances and weighting was based on the number of observations minus 1 for each patient. The model took into account the situation where patients had data on both agents and could therefore act as their own control. RESULTS: The mean within-patient variance in haemoglobin levels for pre-dialysis patients receiving darbepoetin alfa was 63% (95% CI: 1% to 163%) greater than that for patients receiving epoetin alfa (p=0.045). The mean haemoglobin level for patients receiving darbepoetin alfa was 10.34 g/dL (95% CI: 9.94, 10.80) compared with 11.22 g/dL (95% CI: 10.95, 11.50) for patients receiving epoetin alfa (p=0.001). For peritoneal dialysis patients, those administered darbepoetin alfa had similar within-patient variability in haemoglobin levels compared to those receiving epoetin alfa (darbepoetin 10% greater than epoetin alfa, 95% CI: -28% to 66%, p=0.7). The mean haemoglobin level for patients receiving darbepoetin alfa was 11.49 g/dL (95% CI: 11.19, 11.78) compared with 11.16 g/dL (95% CI: 10.81, 11.50) for patients receiving epoetin alfa (p=0.14). CONCLUSION: In spite of the small sample size, there was a significantly greater within-patient fluctuation in haemoglobin levels in pre-dialysis patients receiving darbepoetin alfa compared with epoetin alfa, while no difference was seen in peritoneal dialysis patients. The implications of haemoglobin fluctuations on patient outcomes and resource use require further study.