Background: Granulocyte colony stimulating factor (G-CSF) given for 4–6 days is commonly used for mobilization of allogeneic stem cell donors. The primary objective of this study is to compare the yield of stem cell mobilization, assessed using a surrogate endpoint of CD34+ cell count, between Day 4 and Day 6. Study design and Methods: In this retrospective study we included all allogeneic stem cell donors mobilized with G-CSF for 6 days from January 2003 until October 2015 in the bone marrow transplantation unit at a tertiary academic center. Of 106 donor records reviewed, 84 were with available data and selected for the study. Results: We included 84 donors with median age and weight of 19 years and 60 kg respectively. The median Day 4 WBC and CD34+ cell count were 37.4 × 109/L and 54 × 106/L respectively; while the median Day 6 WBC and CD34+ cell count were 44.4 × 109/L and 86 × 106/L respectively with a statistically significant difference from Day 4 (P < 0.001). In the multivariable model, there were no significant impact of donor's age (P = 0.215), weight (P = 0.108), height (P = 0.428) and mean corpuscular volume (P = 0.263) on the difference in CD34+ cell yield. However, the donor's blood group AB predicated a significantly higher difference (P = 0.036). Conclusion: Six days of G-CSF mobilization achieves higher CD34+ cell count than 4 days in allogeneic stem cell donors especially in donors with blood group AB, albeit both approaches give count higher than the successful collection threshold.