Febrile neutropenia is a complication of myleotoxic chemotherapy that can markedly decrease quality of life and increase healthcare costs. A granulocyte-colony stimulating factor (G-CSF) is used to increase neutrophil production to reduce the risk of developing febrile neutropenia. However, G-CSF administered on the same day as chemotherapy can worsen and prolong neutropenia. To study and compare the effects of pegfilgrastim on the incidence of febrile neutropenia and grade 4 neutropenia in patients receiving pegfilgrastim on the same day (day 1) versus the next day (day 2 or beyond) of chemotherapy, a retrospective, single-center, nonrandomized, cohort study was carried out of adult non-Hodgkin's lymphoma patients who received pegfilgrastim 6 mg subcutaneously on day 1 or beyond of CHOP (cyclophosphamide, doxorubicin, vincristine, prednisone) with or without rituximab every 3 weeks. Six hundred and fifty-five chemotherapy cycles (320 cycles for the same day and 335 cycles for the next day) were evaluable in 141 patients. Among all cycles, the incidence of febrile neutropenia was 9.4 versus 5.1% in the same-day versus the next-day group (P=0.03). The incidence of febrile neutropenia was the highest after the first cycle in the same-day group, which was 19.4, versus 11.1% for the next-day group (P=0.27). There were three deaths among patients who developed febrile neutropenia, including two in the next-day group versus one in the same-day group. In conclusion, our findings support the need for a randomized phase III study to fully evaluate whether a G-CSF is safer when administered on the next day versus the same day of chemotherapy.