Inclusion Criteria Patients must meet the following criteria for study entry: * Written informed consent from parent/legal guardian and willingness of parent/legal guardian to abide by the requirements of the study * Written informed consent or assent from child where appropriate If required by national legislation, patients <18 years of age at screening who are legally considered to be adults according to national legislation must consent in their own right. * Pediatric patients 3 months - 17 years of age with clinically stable chronic renal anemia * CKD with estimated glomerular filtration rate (eGFR) of < 45 mL/min/1.73 m2 (determined by the Bedside Schwartz formula) or dialysis treatment for at least 8 weeks before the first dose of Mircera * For patients on peritoneal dialysis (PD): a weekly Kt/V >= 1.8 * For patients on HD: adequate HD, urea reduction ratio (URR) > 65% or Kt/V > 1.2 for patients on HD three times per week. Patients with fewer than or more than three HD sessions per week should have a weekly Kt/V >= 3.6. * Baseline Hb concentration 10.0 - 12.0 g/dL determined from the mean of two Hb values measured at Visit 1 (Week - 3) and Visit 2 (Week - 1) * Stable SC maintenance treatment with epoetin alfa, epoetin beta, or darbepoetin alfa with the same dosing interval for at least 6 weeks before the first dose of Mircera * Stable dose of epoetin alfa, epoetin beta, or darbepoetin alfa treatment with no weekly dose change > 25% (increase or decrease) for at least 4 weeks before the first dose of Mircera * Adequate iron status defined as ferritin >=100 ng/mL or transferrin saturation (TSAT) >= 20% (or percentage of hypochromic red cells < 10%); mean of two values measured during screening * For post-pubertal female patients of childbearing potential: agreement to remain abstinent (refrain from heterosexual intercourse) or to use acceptable contraceptive methods during the study and for 90 days after the final dose of Mircera A female patient is considered to be of childbearing potential if she is postmenarcheal. The following are acceptable contraceptive methods: hormonal contraceptives; hormone-releasing intrauterine devices; copper intrauterine devices; male or female condom with or without spermicide cap, diaphragm, or sponge with spermicide. * The reliability of sexual abstinence should be evaluated in relation to the duration of the clinical trial and the preferred and usual lifestyle of the patient. Periodic abstinence (e.g., calendar, ovulation, symptothermal, or postovulation methods) and withdrawal are not acceptable methods of contraception.