Abstract Background: Febrile neutropenia (FN) is a common side effect of myelosuppressive chemotherapy. Primary prophylaxis with colony-stimulating factors (CSFs) can reduce FN incidence and is recommended when a patient has a high risk of FN (> 20%). In the prophylactic setting, CSFs should be administered at least 24 hours after chemotherapy completion. Patient burden associated with CSF administration is not well understood. Here we describe current patterns of chemotherapy use and burden of CSF injections for patients with early-stage breast cancer (ESBC) in US clinical practice. Methods: This was a prospective cohort study of adult ESBC patients receiving their first chemotherapy course who had a high risk of FN based on high- or intermediate-risk chemotherapy regimen and individual FN risk factors. The burden associated with CSF injections was assessed via questionnaires among patients who received CSF, and a subset analysis of patient burden in the first cycle of chemotherapy is reported. Results: 598 patients completed the "burden of CSF injections" questionnaire following the first cycle of chemotherapy. Most patients were < 65 years old (76.8%), had a BMI < 30 kg/m2 (54.9%), and had few comorbidities (see table for additional characteristics and comorbidities). The three most common chemotherapy regimens received were ddAC-T (34.4%), TC (23.4%), and TCH (15.6%). 98.3% of patients received prophylaxis with CSF in the first chemotherapy cycle: 94.6% of these received pegfilgrastim, and 5.4% received filgrastim. Among all patients who received CSF, mean (SD) one-way travel time for a single CSF injection was 31 (25) minutes; mean (SD) time in office to receive a CSF injection was 41 (68) minutes. Across the first chemotherapy cycle, mean (SD) time missed from work for CSF administration was 3.1 (9.3) hours, and mean (SD) time missed from non-work activities was 5.5 (14.4) hours. 66.3% of patients had someone else assist them with travel to the clinic to receive CSF, of which 98.8% were helped by an unpaid caregiver. When patients were questioned about the subjective burden of CSF injections, 25.4% reported some degree of bother, and 15.9% reported at least moderate inconvenience. Conclusions: Among the high- and intermediate-risk regimens investigated here, dose-dense and taxane-based chemotherapy regimens were common. As many high-risk patients with ESBC receive primary prophylaxis with CSF, travel and time needed to receive CSF can contribute to patient and caregiver burden. Patient CharacteristicsN = 598 Age, mean (SD) years55.1 (11.3) BMI, mean (SD) kg/m230.7 (7.5) HER2+23.2% Luminal Aa52.0% Triple negative21.6% Stage at diagnosis 124.6% 253.7% 321.2% Missing0.5%Comorbidities > 10% Hypertension37.5% Hyperlipidemia24.7% Depression13.9% Diabetes mellitus13.4% Osteoarthritis12.7% Gastroesophageal reflux disease12.2% Hypothyroidism12.0% Anxiety11.9%aHormone receptor positive but HER2 negative. Citation Format: Li X, Barron R, Tzivelekis S, Li Y, Chandler D, Xu H, Morrow PK, Klippel Z, Nagarajan A, Reiner M, Page JH. A prospective study of patterns of chemotherapy, colony-stimulating factor use, and burden of colony-stimulating factor injections in patients with early-stage breast cancer. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P1-10-04.