The optimal duration of filgrastim as primary febrile neutropenia (FN) prophylaxis in early breast cancer (EBC) patients is unknown, with 5, 7 or 10 days being commonly prescribed. This trial evaluates whether 5 days of filgrastim was non-inferior to 7/10 days.In this randomised, open-label trial, EBC patients who were to receive filgrastim as primary FN prophylaxis were randomly allocated to 5 vs 7 vs 10 days of filgrastim for all chemotherapy cycles. A protocol amendment in November 2017 allowed subsequent patients (N=324) to be randomised to either 5 or 7/10 days. The primary outcome was a composite of either FN or treatment-related hospitalisations. Secondary outcomes included chemotherapy dose reductions, delays, and discontinuations. Analyses were performed by per protocol (primary) and intention-to-treat and the non-inferiority margin was set at 3% for the risk of having FN and/or hospitalisation per cycle of chemotherapy.Patient (N=466) were randomized to receive 5 (184, 39.5%), or 7/10 (282, 60.5%) days of filgrastim. In our primary analysis, the difference in risk of either FN or treatment-related hospitalisation per cycle was -1.52% (95%CI: -3.22% to 0.19%) suggesting non-inferiority of a 5-day filgrastim schedule compared to 7/10-days. The difference in events per cycle for FN was 0.11% (95%CI: -1.05 to 1.27) while for treatment-related hospitalisations it was -1.68% (95%CI: -2.73% to -0.63%). The overall proportions of patients having at least one occurrence of either FN or treatment-related hospitalization were 11.8% and 14.96% for the 5- and 7/10-day groups respectively (Risk Difference: -3.17%, 95%CI: -9.51% to 3.18%).Five days of filgrastim was non-inferior to 7/10 days. Given the cost and toxicity of this agent, 5 days should be considered standard of care.