Preapheresis quantification of circulating CD34+ cells (CD34) may be used to predict peripheral blood progenitor cell (PBPC) yield in subsequent leukapheresis products. In this study, we retrospectively analyzed the results obtained from 113 PBPC leukapheresis procedures performed on two groups of patients (49 adults and 21 children) with malignant blood diseases. CD34 were analyzed by flow cytometry on blood samples taken immediately prior to leukapheresis and on the resulting harvest products. In both adults and children, preapheresis CD34/ml peripheral blood highly correlated with leukapheresis CD34/kg patient body weight (r=0.94, P < 0.001). Regardless of the patient's diagnosis or mobilization regimen, preapheresis blood samples containing >-34.6 x 10³ CD34/ml in adults and >-17.6 x 10³ CD34/ml in children provided more than 2 x 10⁶ CD34/kg in 81% and 88.5% in the leukapheresis products respectively. Our results confirm that quantification of CD34 in preapheresis blood samples on the day of harvest is a reliable guide for predicting progenitor cell number in subsequent harvest products.