This single arm study will evaluate the maintenance of hemoglobin levels, safety and tolerability of once-monthly intravenous administration of Mircera in dialysis patients with chronic renal anemia. Patients will receive intravenous Mircera (120, 200 or 360 micrograms) every four weeks depending on the previous dose of epoetin alfa administered in the week preceding first study drug administration. Patients will be treated for 12 weeks with follow up 2 weeks after the last treatment visit. The anticipated time on study treatment is 3-12 months, and the target sample size is 100-500 individuals.