PURPOSE: In children, at least two or more stem cell mobilization processes are needed in autologous peripheral blood stem cell transplantation to prevent delayed engraftment. And to decrease the risk of tumor cell contaminations, the use of CD34 positive cell selcetion is in increasing tendency. The first leukapheresis product is stored overnight and undergoes CD34 positive selection process mixed with the next day leukapheresis product to save the costs. We intended to find out the optimal overnight storage condition that might minimize the loss of stem cell components. METHODS: RBC (red blood cell)- depleted human umbilical cord bloods (UCB) were used as the source of stem cells because of their easy availability. UCB were processed by isolating the mononuclear cell (MNC) layer using Ficoll-Paque to make the nature similar to leukapheresis products. Fifteen individual UCB were analyzed by several parameters (MNC count and viability, live CD34 positive cell fraction, clonogenic potential) at fresh conditions and under four different overnight storage conditions (room temperatiure (RT), room temperature with autoplasma (AP), 4degrees C, 4degrees C with autoplasma). Analysis of variance, Kruskal-Wallis test, and Wilcoxon signed rank test were used for statistical analyses. RESULTS: Though MNC counts were statistically not different between each conditions (P=0.07), the best recovery (mean 86.9%) was observed at 4degrees C with AP but without statistical significance. MNC viability decreased at RT with or without AP (P<0.05). On the other hand, no difference in MNC viability was noted at 4degrees C with or without AP (P> 0.05). Live CD34 positive cell fractions were significantly decreased under all four different storage conditions compared with fresh ones. However, the samples stored at 4degrees C showed less prominent decreases in live CD34 positive cell fractions than those of RT conditions irrespective of the presence of AP (P=0.0001). CONCLUSION: It seems that 4degrees C condition is superior to RT when short term storage of stem cell products is mandatory. The addition of AP seemed to be advantageous but without statistical significance. The overnight storage of stem cell products at 4degrees C seems to be mandatory because it offers relatively high recovery and less loss of stem cell components. Although the effect of AP was statistically not significant, the role of AP should be studied further because there was a tendency of higher recovery of stem cells in the presence of AP.