Chemotherapy plays an important role in the treatment of early breast cancer (EBC). Granulocyte-colony stimulating factors (G-CSF) can reduce the risk of febrile neutropenia as primary prophylaxis (PP) or secondary prophylaxis (SP). The BRONS study investigated the incidence of serious neutropenic events (SNE) and G-CSF use in a Belgian population of EBC patients treated with myelosuppressive polychemotherapy. Conducted in 2011, this study was a prospective, multicentre, observational trial involving 260 patients. The primary endpoint was the incidence of SNE defined as either febrile neutropenia (FN) or prolonged severe neutropenia (PSN; neutrophil count ≤0.5 × 10⁹ for at least five days). Secondary endpoints included a description of the chemotherapeutic regimens prescribed and G-CSF use. Nine percent of patients were treated with a dose-dense regimen (DD) and 91% received classical chemotherapy (CC). PP with G-CSF (PPG) was given to 20% of patients (100% in DD and 11% in CC). Eighteen percent of patients presented a SNE (4% in DD and 20% in CC) of which 15% were FN and 3% PSN. SNE occurrence was 8% in the PPG subgroup and 21% in the no-PPG subgroup. In the DD subgroup, all patients received PPG and no FN was reported. Twenty six adverse events related to G-CSF were reported in 8.2% of patients and two of these were classified as severe. This observational study highlights the high incidence of SNE with CC regimens in patients who do not receive PPG. It also confirms the safe profile of DD regimens with G-CSF support.