PURPOSE: Myelosuppression, particularly neutropenia, is one of the most frequent and serious toxicity seen in patients with breast cancer undergoing systemic chemotherapy. However, the predictive factors for development of severe neutropenia in chemotherapy remain unknown. We therefore evaluated predictive factors for excessive myelosuppression.METHODS: We retrospectively analyzed 341 patients with breast cancer treated with chemotherapy from 2000 to 2012. Clinicopathological characteristics, number of using of granulocyte colony-stimulating factor (G-CSF), and pretreatment hematologic values were extracted from the electronic medical record system. Patients were sorted 2 groups by number of using G-CSF in each chemotherapeutic regimens; group 1 is more G-CSF (within high 20 percentile) and 2 less G-CSF using group (within lower 20 percentile).RESULTS: Number of using G-CSF was ranged 0–83 (mean 10.76). One hundred one patients were in group 1 and 65 patients were in group 2. Mean of number of G-CSF using was 0.21 in group 1 and 28.02 in group 2. Pretreatment white blood cell, hemoglobin and platelet count were lower in group 2 than in group 1 (6.88×10³/µL vs. 5.97×10³/µL, 12.63 g/dL vs. 11.90 g/dL, and 275.95×10⁴ µL vs. 227.37×10⁴ µL). There were no statistically differences in other clinicopathologic characteristics such as age, body mass index or comorbidities, hormonal receptor, stage, and other pretreatment hematologic values.CONCLUSION: Pretreatment white blood cell count, hemoglobin and platelet count can be used to identify patients at increased risk of significant myelosuppression undergoing chemotherapy with breast cancer. This information can be used to target high-risk patients for prophylactic treatment.