Background: Selection of adjuvant breast cancer (BC) therapy varies based on patient risk factors. Choice of chemotherapy (CT), schedule, duration, and supportive care affects costs and toxicity. Because pegfilgrastim (PEG) is a large driver of cost in adjuvant therapy it is important to understand the utilization characteristics. We aimed to characterize PEG utilization during neoadjuvant/adjuvant (N/Ad) CT by age and regimen type. Methods: Using the US Oncology iKnowMed EHR database, we retrospectively identified female BC patients (pts) diagnosed with stage I-III BC, between 7/2006 and 11/2010. Secondary diagnoses were excluded. Pts were characterized by age, ER and HER2 status, tumor size, grade, and nodes. CT utilization was determined by the number of pts assigned an N/Ad line of therapy (LOT) during the study period. Regimens were categorized by CT title and drugs. Clinical trial pts were included. PEG utilization was characterized if administered within 6 months of being assigned to an N/Ad CT regimen. Results: During the time period, 40,881 newly diagnosed localized BC pts were identified. Of these, 15,328 pts (37%) were assigned an N/Ad CT regimen and 72% (11, 022 pts) received PEG at any time within 6 months of their N/Ad CT. When analyzed by regimen type, TC, TAC, TCH, and dose-dense AC-paclitaxel were the most highly utilized regimens accounting for 64% of all PEG use. PEG use with the TC regimen was 70% and represented 25% of all N/Ad PEG utilization. PEG utilization by age was characterized as 81% of pts <40 yrs, 74% of pts 40-49 yrs, 70% of pts 50-59 yrs, 71% of pts 60-69 yrs, 68% of pts 70-79 yrs, and 40% of pts ≥80 yrs. Conclusions: While 37% of women treated in the N/Ad setting received CT, PEG was given to 72% of that subset. Utilization was high among all age groups. Dose-dense and docetaxel-containing regimens accounted for the majority of PEG use. With availability of generic docetaxel, commonly used drugs in adjuvant BC except trastuzumab will have generic equivalents and PEG will be the largest cost driver in women receiving adjuvant CT in the community. Since febrile neutropenia occurred in 5% of pts on the original TC trial, determination of appropriate level of use of PEG in the adjuvant setting needs further study.