The low number of hematopoietic stem cells (HSC) in umbilical cord blood (UCB) is directly related to increased risk of transplant failure. Effective ex vivo expansion of HSC has been tried for many years, with conflicting results because of the inability to reproduce in vitro HSC proliferation in the same way it occurs in vivo. We compared freshly isolated HSC with their expanded counterparts by microarray analysis and detected activation of the noncanonical Wnt (wingless-type MMTV integration site family) pathway. Study of early alterations during ex vivo UCB-HSC expansion could contribute to improvement of ex vivo expansion systems.