Background: FN is a primary driver of chemotherapy modification and reduced RDI. Breast cancer patients often receive regimens with high risk ( > 20%) for FN. PP G-CSF use is recommended to decrease incidence of infection, as manifested by FN. Real-world evidence describing the association of G-CSF with reduced RDI in breast cancer patients is lacking. Methods: We retrospectively analyzed Flatiron Health Electronic Health Record data from 2011 to 2019 of adult female breast cancer patients initiating high-risk for FN chemotherapy regimens. PP G-CSF (pegfilgrastim, filgrastim, or biosimilars) use was assessed in the first and subsequent cycles of chemotherapy overall and by modality (on-body injector [OBI] vs. pre-filled syringe [PFS]). Standard dose was defined by NCCN guidelines. RDI was defined with respect to dose and timing of delivery: (delivered dose intensity/standard dose intensity)*100 and categorized as < 85% vs ≥85%. Characteristics were described by G-CSF use and adjusted relative risks (ARRs) were estimated for the association between G-CSF use and RDI, adjusting for age, serum hemoglobin, ECOG, HER2 status, and HR status. Results: There were 27,812 breast cancer patients included with a median age of 57 years. Most (90%) received PP-G-CSF. Four cycles of TC (docetaxel + cyclophosphamide) was the most common regimen prescribed (36%) and 82% of these patients received PP-G-CSF support in the first cycle, with OBI given more often than PFS (62% vs 38%). Second most common was dd AC-T Q1W (doxorubicin + cyclophosphamide followed by paclitaxel) and 98% of these patients received PP G-CSF (59% OBI vs 41% PFS). 80% of G-CSF users (vs 48% of those with no G-CSF) completed 4 or more cycles of chemotherapy (all regimens had ≥4 planned cycles). G-CSF users had a significant reduction in risk of RDI < 85% in Cycle 1 (ARR: 0.60; 95% confidence interval [CI]: 0.43 – 0.85) and Cycle 2 (ARR: 0.76; 95% CI: 0.59 – 0.97). At the overall chemotherapy course level, the risk of RDI < 85% was not significantly different by G-CSF use (ARR: 0.83; 95% CI: 0.63 - 1.08). There was no significant difference in risk of reduced RDI between OBI and PFS (ARR: 1.13; 95% CI: 0.94 - 1.35). Conclusions: This study revealed that 90% of breast cancer patients receiving high-risk for FN chemotherapy received PP-G-CSF support and completed more cycles than those who did not receive G-CSF. G-CSF users had lower risk of a reduced RDI in Cycles 1 and 2.