Introduction: Biosimilars of granulocyte colony-stimulating factors (G-CSF) have shown similar efficacy to originator filgrastim (Neupogen [NEU]; Amgen Inc.) as prophylaxis in neutropenia and in the mobilization of stem cells in patients receiving combination chemotherapy with G-CSF. Methods: This was a retrospective study in which the characteristics of stem cell mobilization treated with a G-CSF alone were compared in 216 patients and 56 donors. The two G-CSF compared were NEU and the biosimilar filgrastim Zarzio (Sandoz GmbH) (referred to hereafter as BIO). Primary objectives were mobilization rate (minimum of 10 9 103/ml CD34+ on day 4 of treatment [day +4]) and use of the immunostimulant plerixafor (PLEX) in each group. Results: The general characteristics of the patients receiving NEU (n = 138) and those receiving BIO (n = 78) did not differ significantly. PLEX was used in 24% of BIO patients and in 25.7% of NEU patients. The median CD34+ cell count on day +4 was significantly lower in BIO patients who needed PLEX than in those who did not (2.4 vs. 4.8 9 103 /ml; p = 0.002), as was the final CD34+ cell count (2.5 vs. 3.3 9 106 /kg; p 0.03). Mobilization failure rate was higher in the BIO group than in the NEU group (20 vs. 0%; p = 0.01). With respect to donors, more than one apheresis was needed in three BIO donors, one of them with PLEX. The use of BIO was the only risk factor for mobilization failure in patients who needed PLEX (hazard ratio 10.3; 95% confidence interval 1.3-77.8). Conclusion: The study revealed that BIO had a lower efficacy for stem cell mobilization when the only treatment was G-CSF, especially in poor mobilizers needing PLEX.