['In bone marrow, hematopoietic stem cells (HSCs) reside in the most hypoxic endosteum niche, whereas the proliferating progenitors are located near the relatively oxygen-rich vascular region. High oxygen tension is potentially detrimental to HSCs. The objective of this investigation was to compare cellular, functional, and molecular responses of human umbilical cord blood (UCB)-derived hematopoietic stem and progenitor cells in culture under hypoxic and normoxic conditions.', 'CD133-enriched UCB cells were cultured in growth factor containing serum-free and serum-supplemented medium under 5% O(2) (hypoxia) or 21% O(2) (normoxia) for 10 d. The phenotypes of expanded cells were analyzed by flow cytometry and the engraftability by SCID-repopulation assay. The expression of hypoxia-inducible factor (HIF)-1α and some of its target genes was analyzed by real-time RT-PCR.', 'In hypoxic culture, CD34(+) CD38(-) cells were expanded about 27-fold, which was significantly (P < 0.01) higher than that obtained in normoxic culture. Serum-free culture did not support the growth of cells in the presence of 21% O(2) . Myeloid colony-forming potential of cells was significantly (P < 0.05) increased in 5% O(2) compared with 21% O(2) culture. SCID-repopulation efficiency seems to be better preserved in the cells cultured under hypoxic conditions. Hypoxia significantly (P < 0.05) induced the expression of HIF-1α, vascular endothelial growth factor (VEGF), and ABCG2 genes and also upregulated CXCR4 receptor expression.'] Does hypoxia improve expansion potential of human cord blood-derived hematopoietic stem cells and marrow repopulation efficiency?