Optimum storage conditions of cord blood-derived hematopoietic progenitor cells before isolation remain unknown. We therefore evaluated CD34 þ cells isolated from cord blood units (n ¼ 57) within 1 h after collection and following storage for 24, 48 and 72 h at either room temperature (RT) or 4 1C. Isolated CD34 þ cells were analyzed for their cell count, immunophenotype, apoptosis rate, clonogenicity and transmigration capacity in response to stromaderived factor 1a using direct-paired comparisons (n ¼ 27). CD34 þ , CD133 þ and CD45 þ positivity after isolation remained the same under all conditions. After 24 h, CD34 þ cell counts and numbers of CFU-GM colonies dropped regardless of the storage temperature. After 48 h, the number of CD34 þ cells increased compared to 24 h, if the cord blood had been stored at RT resulting in almost three times more CD34 þ cells than at 4 1C. These cells had a lower early apoptosis rate and formed four times more BFU-E than those stored at 4 1C with equivalent plating efficiencies. CD34 þ 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 þ isolation and a storage time for 48 h should be preferred to 24 h.