Adult human bone marrow (ABM) is an important source of hematopoietic stem cells for transplantation in the treatment of malignant and nonmalignant diseases. However, in contrast to the recent progress that has been achieved with umbilical cord blood, methods to expand ABM stem cells for therapeutic applications have been disappointing. In this study, we describe a novel culture method that uses human brain endothelial cells (HUBECs) and that supports the quantitative expansion of the most primitive measurable cell within the adult bone marrow compartment, the nonobese diabetic/severe combined immunodeficient (NOD/SCID) repopulating cell (SRC). Coculture of human ABM CD34⁺ cells with brain endothelial cells for 7 days supported a 5.4-fold increase in CD34⁺ cells, induced more than 95% of the CD34⁺CD38⁻ subset to enter cell division, and produced progeny that engrafted NOD/SCID mice at significantly higher rates than fresh ABM CD34⁺ cells. Using a limiting dilution analysis, we found the frequency of SRCs within fresh ABM CD34⁺ cells to be 1 in 9.9 × 10⁵ cells. Following HUBEC culture, the estimated frequency of SRCs increased to 1 in 2.4 × 10⁵cells. All mice that received transplants of HUBEC-cultured cells showed B-lymphoid and myeloid differentiation, indicating that a primitive hematopoietic cell was preserved during culture. Noncontact HUBEC cultures also maintained SRCs at a level comparable to contact HUBEC cultures, suggesting that cell-to-cell contact was not required. These data demonstrate that human brain endothelial cells possess a unique hematopoietic activity that increases the repopulating capacity of adult human bone marrow.