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This is an updated systematic review of 57 trials and 9353 cancer patients from articles, abstracts, and reports published between January 1, 1985, and April 30, 2005, on the effects of epoetin alfa and beta (i.e., epoetin) and darbepoetin alfa (i.e., darbepoetin). We included randomized controlled trials comparing epoetin or darbepoetin plus red blood cell transfusion with red blood cell transfusion alone for prophylaxis or treatment of anemia in cancer patients with or without concurrent antineoplastic therapy. The Cochrane Library, MEDLINE, EMBASE, and conference proceedings were searched. Effect estimates and 95% confidence intervals (CIs) were calculated with fixed-effects models. Treatment with epoetin or darbepoetin statistically significantly reduced the risk for red blood cell transfusions (relative risk [RR] = 0.64, 95% CI = 0.60 to 0.68; 42 trials and 6510 patients) and improved hematologic response (RR = 3.43, 95% CI = 3.07 to 3.84; 22 trials and 4307 patients). Treatment with epoetin or darbepoetin increased the risk of thrombo-embolic events (RR = 1.67, 95% CI = 1.35 to 2.06; 35 trials and 6769 patients). Uncertainties remain as to whether and how epoetin or darbepoetin affects overall survival (hazard ratio = 1.08, 95% CI = 0.99 to 1.18; 42 trials and 8167 patients). Caution is advised when using epoetin or darbepoetin in combination with thrombogenic chemotherapeutic agents or for cancer patients who are at high risk for thrombo-embolic events.