Adrenoleukodystrophy (ALD) is a demyelinating disease of the central nervous system that results from a genetic deficiency of ALDP, an ABC protein involved in the transport of very long-chain fatty acids (VLCFAs). The cloning of the ALD gene and the positive effects of allogeneic bone marrow transplantation support the feasibility of a gene therapy approach. We report the retroviral transfer of the ALD cDNA to peripheral blood and bone marrow CD34+ cells from control donors and ALD patients. Prestimulation of these cells with cytokines, followed by infection with the M48-ALD retroviral vector, resulted in 20% transduction efficiency (4-40%) and expression of the vector-encoded ALDP in 20% of CD34+ cells (7.3-50%). Long-term culture (LTC) of transduced CD34+ cells from two ALD patients showed efficient transduction (24-28%) and stable expression (25-32%) of ALDP in derived clonogenic progenitors at 3 weeks of culture. The expression of ALDP in CFU cells derived from 5 and 6 weeks of LTC confirmed the effective transduction of LTC-initiating cells. Expression of ALDP was observed in CD68+ CFU-derived cells, suggesting that monocyte-macrophages, the target bone marrow cells in ALD, were produced from transduced progenitor cells. VL-CFA content was corrected in LTC and CFU-derived cells in proportion to the percentage of transduced cells, indicating that the vector-encoded ALDP was functional. Although not efficient yet to allow a clinical perspective, these results demonstrate the feasibility of ALD gene transfer into CD34+ cells of ALD patients.