Optimum storage conditions of cord blood-derived hematopoietic progenitor cells before isolation remain unknown. We therefore evaluated CD34+ cells isolated from cord blood units (n=57) within 1 h after collection and following storage for 24, 48 and 72 h at either room temperature (RT) or 4 degrees C. Isolated CD34+ cells were analyzed for their cell count, immunophenotype, apoptosis rate, clonogenicity and transmigration capacity in response to stroma-derived factor 1alpha using direct-paired comparisons (n=27). CD34+, CD133+ and CD45+ positivity after isolation remained the same under all conditions. After 24 h, CD34+ cell counts and numbers of CFU-GM colonies dropped regardless of the storage temperature. After 48 h, the number of CD34+ cells increased compared to 24 h, if the cord blood had been stored at RT resulting in almost three times more CD34+ cells than at 4 degrees C. These cells had a lower early apoptosis rate and formed four times more BFU-E than those stored at 4 degrees C with equivalent plating efficiencies. CD34+ cells kept at RT for 48 h had the highest transmigration capacities, which paralleled an increased CXCR-4 expression. Cord blood should be stored at RT before CD34+ isolation and a storage time for 48 h should be preferred to 24 h.