Background: Biosimilars have potential to reduce healthcare costs and increase access in the United States, but lack of uptake has contributed to lost savings. Filgrastim-sndz was the first FDA-approved biosimilar, and much can be learned by evaluating its uptake. In February 2016, the US Oncology Network converted to filgrastim-sndz as its short-acting granulocyte colony-stimulating factor (GCSF) of choice for prevention of febrile neutropenia (FN) following myelosuppressive chemotherapy (MCT). To understand utilization and cost patterns, this study analyzes real-world data of GCSFs within a community oncology network during the initial period of conversion to the first biosimilar available in the US. Methods: This descriptive retrospective observational study used electronic health record data for female breast cancer (BC) patients receiving GCSF and MCT at high risk of FN. Patient cohorts were defined by first receipt of either filgrastim or filgrastim-sndz during the 410 days before and after biosimilar conversion. Healthcare resource utilization (HCRU) and costs for GCSF and complete blood counts (CBC) were collected at GCSF initiation through the earliest of 30 days following end of MCT, loss to follow up, death, or data cutoff. Results: 146 patients were identified: 81 (55.5%) filgrastim and 65 (44.5%) filgrastim-sndz. No directional differences existed in baseline characteristics between the cohorts. Higher proportions of filgrastim-sndz patients received dose-dense MCT (33.8% vs 22.2%). Time trends show an initial spike in HCRU and cost for filgrastim-sndz patients after formulary conversion, which subsequently decreased and converged to that of the filgrastim cohort after 12 months. When aggregated, the overall median total administration counts, per patient per month (PPPM) and dosage, were marginally higher for filgrastim-sndz (5 vs 3; 2.9 vs 1.4; 1920 vs 1440 mcg, respectively). Median PPPM costs were higher for filgrastim-sndz ($803 vs $545). Median CBC utilization and costs were higher for filgrastim-sndz (2.8 vs 2.5; $28 vs $23, respectively). Conclusions: This study provides insight into real-world HCRU and cost patterns after formulary conversion to a biosimilar for BC patients receiving MCT and GCSF. As a descriptive study, causal inferences cannot be made and an underlying effect from index chemotherapy cannot be excluded. Convergence of HCRU and costs after 12 months suggests that overall results may be driven by behavior at initial formulary switch. Since filgrastim-sndz was the first US biosimilar approved, the uptake may be indicative of an experience with biosimilar acceptance in general. Future real-world studies of biosimilars must consider inconsistent utilization and practice trends during the time frame directly following formulary conversion.