We evaluated the optimal conditions for blood sampling for hematopoietic progenitor cells (HPCs) as estimated by the immature information program of the SE-9000 automated hematology analyzer. The HPC count was most stable when the blood samples were incubated at room temperature with ethylene-diaminetetraacetic acid dipotassium (EDTA-2K) as an anticoagulant. The HPC count should, however, be measured within 4 h after blood collection, even under optimal conditions. In contrast, the CD34+ cell count estimated by flow cytometric analysis was stable for at least 21 h after the blood samples were incubated with EDTA-2K at room temperature or 4°C. When appropriate blood samples were used, the HPC count in the peripheral blood significantly correlated with the CD34+ cell count in the peripheral blood and in the apheresis yields (r = 0.798 and 0.635, respectively); therefore, the HPC count is a reliable predictor for initiation of apheresis procedures to obtain sufficient HPCs for peripheral blood stem cell transplantation.