Purpose: Prophylactic use of granulocyte colony-stimulating factor (G-CSF) is recommended for cancer patients who are at high risk of neutropenic events. However, whether the clinical effectiveness of G-CSF from randomized controlled trials translates into "realworld" clinical practice is questionable. The goal of this retrospective cohort study was to examine the impact of G-CSF prophylaxis and other potential risk factors of severe neutropenia in women with breast cancer. Methods: Our study subjects were women who were diagnosed with breast cancer and who received a new course of chemotherapy between January 1, 2010, and December 31, 2010, at a cancer center in Taiwan. Generalized estimating equations were applied to examine the association between G-CSF prophylaxis and neutropenic events. Findings: We identified 353 women with breast cancer who received a total of 2776 cycles of chemotherapy. G-CSF was used as primary prophylaxis in 7% (n = 202) of cycles and as secondary prophylaxis in 11% (n = 319) of cycles. The mean duration of G-CSF for primary and secondary prophylaxis was 4.9 and 3.7 days, respectively. A chemotherapy regimen with high risk of febrile neutropenia was found to be a risk factor for severe neutropenic events (odds ratio, 3.22 [95% CI, 1.97-5.27]). Prophylactic use of G-CSF was not statistically significantly associated with febrile neutropenia. Implications: The major determinants of neutropenic events among patients with breast cancer were the content and intensity of chemotherapy regimens. Suboptimal use of G-CSF may not be effective in preventing neutropenic events among women with breast cancer.