Background: The National Comprehensive Cancer Network (NCCN) recommends several chemotherapy regimens for early-stage breast cancer that have elevated risk of neutropenia (>20%). In these regimens, primary prophylactic use of granulocyte colony-stimulating factor (G-CSF) is recommended. Little is known about how patient-level factors relate to receipt of prophylactic G-CSF among women receiving these regimens. Methods: In the Optimal Breast Cancer Chemotherapy Dosing (OBCD) Study, we examined use of G-CSF among women with stage I-IIIA breast cancer between 2005-2019 in Kaiser Permanente Northern California (KPNC) and Kaiser Permanente Washington (KPWA). We evaluated the patient-level factors associated with primary prophylactic G-CSF use (≤7 days before to ≤3 days after first day of chemotherapy) among women receiving chemotherapy regimens with high risk of neutropenia, for which the NCCN guidelines recommend prophylactic G-CSF use. Multivariable-adjusted prevalence ratios (PR) and their corresponding 95% Confidence Intervals (95% CI) were calculated using generalized linear models of the Poisson family with a log link-function and robust standard errors. Results: Overall, while all 3,083 women receiving these regimens ultimately received G-CSF, 2412 (79%) received primary prophylactic G-CSF within the period 7 days prior to 3 days post first cycle. Younger women had higher prevalence of prophylactic G-CSF receipt (82.7% in ages 18-39 years vs. 58.1% in 70+ years). Black women had higher prevalence of receipt compared to white women (82.7% in non-Hispanic Black women vs 76.1% in White women). Women with a higher body mass index (BMI) had lower prevalence of receipt compared to normal weight women (73.3% in 35+ kg/m2 group vs 81.5% in 18.5-<25 kg/m2 group). In multivariable models, older women were less likely to receive prophylactic G-CSF (PR_{70+ vs 18-39}: 0.69; 95% CI: 0.57-0.84; p-trend<0.001), as were those with a higher BMI (PR_{BMI 35+ vs 18.5-<25}: 0.92; 95% CI: 0.85, 0.97; p-trend=0.006), and a later year of diagnosis (PR_{2016-19 vs 2005-07}: 0.81; 95% CI: 0.77, 0.85; p-trend<0.001). Although race/ethnicity was not significantly associated with receipt of prophylactic G-CSF use overall in multivariable models, we observed Non-Hispanic Black women to experience slightly higher likelihood of G-CSF receipt than Non-Hispanic White women (PR: 1.08; 95% CI: 1.02, 1.15). Conclusions: Women of older age and those with higher BMI were less likely to receive primary prophylactic G-CSF, as were women treated in later years. Further research looking at timing of G-CSF use is warranted, as are studies examining provider/facility-level factors in relation to patterns of G-CSF use, G-CSF’s impact on survival outcomes, and the real-world impact of decisions around G-CSF use across age, BMI, and racial/ethnic groups.