OBJECTIVE To develop a simple method to rapidly determine the optimal time of perpheral blood stem cell harvest. METHODS We selected 5 healthy donors mobilized with granulocyte colony-stimulating factor (G-CSF) in allogenic peripheral blood stem cell transplantation (allo-PBSCT) to continuously monitor the variety of hematopoietic stem/progenitor cells (HSC/HPC) in the peripheral blood. HPC in the peripheral blood was detected by Sysmex XE-2100 fully automated hematology analyzer with an immature myeloid information channel and the CD34+ cells and colony-forming unit-granulocyte-macrophage (CFU-GM) were determined by flow cytometry and colony culture during mobilization. RESULTS HPC was positively correlated with the CD34+ cells or CFU-GM in the peripheral blood (r = 0.82 or r = 0.85, respectively). HPC, CD34+ cells, and CFU-GM in allo-PBSCT donors obviously increased on the 4th day after the mobilization, and their peaks were on the 5th day after the mobilization. But HPC varied more than CD34+ cells and CFU-GM did. CONCLUSIONS Sysmex XE-2100 can rapidly monitor the variety of HSC/HPC in allo-PBSCT donors mobilizated with G-CSF, and can provide a simple and economic method to determine the optimal time of peripheral blood stem cell harvest.