To determine how aging impacts gene expression in hematopoietic stem cells (HSCs), human CD34+ cells from bone marrow (BMCD34+) and mobilized stem cell products (PBCD34+38-) were examined using microarray-based expression profiling. The age-associated expression changes in CD34+ cells were then compared to age-associated expression changes in murine HSCs. Interferon regulatory factor 8 (IRF8) was the only gene with age-associated expression changes in all analyses, decreasing its expression in human CD34+ cells and murine HSCs. Microarray-based expression profiling found that IRF8 expression also decreased with aging in human T-cells, suggesting that the effects of aging on IRF8 expression may extend to more differentiated populations of hematopoietic cells. Quantitative-RT/PCR studies confirmed that IRF8 mRNA expression decreased with aging in additional samples of BMCD34+, PBCD34+38-, and T-cells, and IRF8 protein expression was found to decrease with aging and to correlate with mRNA levels in PBCD34+ cells. The results suggest that IRF8 may be a novel biomarker of aging for hematopoietic cells. Given that inactivation of IRF8 causes CML-like syndromes in mice and decreased IRF8 expression occurs in human hematopoietic malignancies, it will be critical to determine if decreased IRF8 expression plays a role in the increased incidence of hematopoietic malignancies in older adults.