Background: The purpose of this study was to evaluate the incidence of febrile neutropenia (FN) in breast cancer patients with pegylated recombinant human granulocyte-colony stimulating factor (PEG-rhG-CSF) administration at 48 or 24 hours after chemotherapy for primary prevention. Methods: Patients with stage I-III invasive breast cancer who were scheduled to receive 4 cycles of adjuvant chemotherapy with EC regimen (epirubicin combined with cyclophosphamide) were eligible for this multicenter, open-label, randomized trial. Patients were randomized (1:1) to receive PEG-rhG-CSF (Jinyouli) 48 hours (48h group) or 24 hours (24h group) after the end of each cycle of chemotherapy. The primary endpoint was the incidence rate of FN. The secondary endpoints included the incidence rates of grade 3/4 neutropenia, chemotherapy dose reduction and chemotherapy delay due to neutropenia, antibiotic administration, the pain (bone, muscle, or joint), and the relative dose intensity (RDI) of the chemotherapy. Results: The preliminary results enrolled 197 patients including 96 in the 48h group and 101 in the 24h group. 78.1% of 48h group patients and 73.3% of 24h group patients completed four cycles of chemotherapy. The incidence rates of FN were 2.1% in the 48h group and 4.0% in the 24h group respectively. FN occurred in the first two cycles in the 48h group, while in the 24h group, it occurred only in the first cycles. The incidence rate of grade 3/4 neutropenia was 27.1% in the 48h group and 51.5% in the 24h group respectively. There was no delay in chemotherapy due to neutropenia in the two groups. The incidence rate of chemotherapy dose reduction due to neutropenia was 2.1% in the 48h group and 1.0% in the 24h group respectively. 2 (2.1%) patients in the 48h group and 4 (4.0%) patients in the 24h group received antibiotics. The mean RDI of each cycle of chemotherapy was 91.3% in the 48h group and 91.5% in the 24h group, respectively. The incidence rate of all grades pain (bone, muscle, or joint) was 25.0% in the 48h group, and 20.8% in the 24h group. With regard to 3 or higher grade pain, 1 (1.0%) patient in the 48h group experienced bone pain and 1 (1.0%) patient in the 24h group experienced muscle pain. Conclusions: Preliminary results suggest a similar effect of PEG-rhG-CSF administration at 48 or 24 hours after chemotherapy in primary prevention of FN in breast cancer patients receiving EC chemotherapy regimen. Recruitment is ongoing and updated data will be presented. Clinical trial information: ChiCTR1800019516.