The molecular mechanisms underlying hematopoietic stem cell (HSC) aging remain to be elucidated. In this study, we investigated age-related changes in the functional and phenotypic properties of murine HSCs. Consistent with previous studies, we found that the number and frequency of CD34(-/low)c-Kit(+)Sca-1(+)lineage marker(-) (CD34(-)KSL) cells, a highly enriched HSC population, significantly increased in old mice, though their repopulating ability was reduced. Continuous bromodeoxyuridine labeling revealed a significant delay in the cell cycle progression of CD34(-)KSL cells in old mice. This delay was also observed in young recipients transplanted with whole bone marrow cells from old mice. When cultured in vitro, CD34(-)KSL cells from old mice showed a greater capacity to give rise to primitive CD48(-)KSL cells with reduced HSC activity. Gene expression profiling identified age-related changes in the expression of several cell cycle regulatory genes, including p21/Cdkn1a and p18/Cdkn2c. These results support the notion that HSC aging is largely regulated by an intrinsic genetic program.