OBJECTIVES: To better understand the real-world dosing patterns of epoetin alfa (EPO) and darbepoetin alfa (DARB) in patients with cancer receiving chemotherapy, a large retrospective, observational study was undertaken to analyze dosing patterns and associated costs of patients treated with EPO and DARB in the outpatient setting. METHODS: Adult patients who had cancer and were receiving chemotherapy, had ³ 2 EPO or DARB claims, and were newly initiated on erythropoiesis stimulating therapy (EST) between April 2003 and February 2005 were identified from a nationwide sample of outpatient medical claims from hospital clinics and office practices. EPO and DARB use was identified via HCPCS codes in medical claims with dose calculated using billed units. Average treatment duration, dosing frequency, mean cumulative dose, and total EST costs (using 2005 wholesale acquisition prices) were studied. RESULTS: A total of 1,405 EPO and 1,087 DARB patients met the inclusion criteria. Mean age (years; EPO 63±13; DARB 62±12) was similar between groups, with more women in the DARB group (EPO 62%; DARB 71%, p<0.05). Weekly and extended dosing (³Q2W) frequencies were utilized in patients receiving EPO (QW: 65%, Q2W: 29%, ³Q3W: 6%) and DARB (QW: 11%, Q2W: 61%, ³Q3W: 28%). Mean treatment duration was 54±51 days for EPO and 52±46 days for DARB. Mean cumulative dose for EPO and DARB was 296,070 IU and 1,022 mcg corresponding to a dose only ratio of 290:1 (units EPO: mcg DARB) and total EST costs of $3,603 and $4,456, respectively. CONCLUSION: In this large observational study, DARB treatment costs were substantially higher (24%) than EPO treatment costs. In addition, extended EPO and DARB dosing was common among patients with cancer receiving chemotherapy.