Although enumeration of CD34+ cells in the peripheral blood (PB) on the day of apheresis predicts the quantity of those cells collected, the flow cytometric techniques used are complex and expensive, and several hours are required to obtain the result in the clinical practice setting. The Sysmex SE-9000 automated haematology analyzer provides an estimate of immature cells, called hematopoietic progenitor cells (HPC). The aim of this study was to evaluate the clinical usefulness of HPC in predicting the optimal timing of peripheral blood progenitor cells (PBPC) harvest. Studies were performed on 628 aphereses from 160 patients with hematologic or solid malignancies. Spearman's rank statistics was used to assess correlation between HPC, WBC, mononuclear cells (MNC), and CD34+ cells. A receiver operating characteristic (ROC) curve was drawn for cutoff value of HPC, and predictive values of the chosen cutoff value of HPC for different target CD34+ cell collections were calculated. The PB HPC had a stronger correlation (ρ=0.592, <0.001) with collected CD34+ cells than did PB WBC and PB MNC. The ROC curve showed that the best cutoff value of HPC was 50x10⁶/L for the target CD34+ cells ≥1 x10⁶/kg with sensitivity of 75%. Positive and negative predictive values of HPC ≥50x10⁶/L for CD34+ cells ≥1x10⁶/kg were 59.7% and 81.1%, respectively. In the clinical practice setting, applying variable cutoff values of HPC would be a useful tool to predict the optimal timing of PBPC collection.