Background: Chemotherapy takes an important role in mammary cancer treatment, and to obtain therapy effect surely, RDI (relative dose intensity) is important.To preserve RDI, management of febrile neutropenia (FN) is demanded in a side effect. In this country, prolonged model G-CSF is approved, and primary prevention administration was enabled, but examination to relate to choice of the administration patient is not enough. We investigated a risk of FN in patient performed FEC and we examined whether exhaustion of the case that should have considered prolonged model G-CSF administration was possible.Methods: We intended for patient with breast cancer of 186 examples who took FEC medical treatment during from January, 2009 to December, 2014 in our hospital.We analyzed symptom example number that caused FN, patient background, and RDI. We evaluated the adverse event using CTCAE v.4.0.Results: In 186 examples, 10 examples (5.3%) had FN. Preoperative adjuvant therapy 2 of 88 examples (2.3%), postoperative adjuvant therapy 5 of 76 examples (6.5%), recurrence treatment 1 of 9 examples (11.1%), stage4 treatment 2 of 13 examples (15.4%) had FN (P = 0.1). The case that inflammation of mouth was present before FN showed 4 examples. One example had the post-operative wound infection. After FN, 2 examples canceled FEC, and one example made RDI decrease by 80%, and 7 examples maintained RDI 100% (one example uses prolonged model G-SCF together).Conclusion: FN was seen a lot in metastasis recurrence example. It was thought that there was need of mouth care before the FN onset, but we might prevent the onset of FN by prolonged model G-CSF use when it had difficulty to contorol infection. Background: Chemotherapy takes an important role in mammary cancer treatment, and to obtain therapy effect surely, RDI (relative dose intensity) is important. To preserve RDI, management of febrile neutropenia (FN) is demanded in a side effect. In this country, prolonged model G-CSF is approved, and primary prevention administration was enabled, but examination to relate to choice of the administration patient is not enough. We investigated a risk of FN in patient performed FEC and we examined whether exhaustion of the case that should have considered prolonged model G-CSF administration was possible. Methods: We intended for patient with breast cancer of 186 examples who took FEC medical treatment during from January, 2009 to December, 2014 in our hospital.We analyzed symptom example number that caused FN, patient background, and RDI. We evaluated the adverse event using CTCAE v.4.0. Results: In 186 examples, 10 examples (5.3%) had FN. Preoperative adjuvant therapy 2 of 88 examples (2.3%), postoperative adjuvant therapy 5 of 76 examples (6.5%), recurrence treatment 1 of 9 examples (11.1%), stage4 treatment 2 of 13 examples (15.4%) had FN (P = 0.1). The case that inflammation of mouth was present before FN showed 4 examples. One example had the post-operative wound infection. After FN, 2 examples canceled FEC, and one example made RDI decrease by 80%, and 7 examples maintained RDI 100% (one example uses prolonged model G-SCF together). Conclusion: FN was seen a lot in metastasis recurrence example. It was thought that there was need of mouth care before the FN onset, but we might prevent the onset of FN by prolonged model G-CSF use when it had difficulty to contorol infection.