The potential of granulocyte colony-stimulating factor (G-CSF) to mobilize CD4 cells and/or CD34 cells for use in gene therapy or to support antiretroviral therapy was examined. Ten human immunodeficiency virus-infected patients were treated with G-CSF (300 microg/day) for 5 days. Numbers of CD4 and CD34 cells were measured. To examine the numbers of naive and memory type CD4 cells, CD4 cell coexpression of CD45RA and CD45RO was measured. Functionality of mobilized CD4 cells was examined by use of the proliferation assay and interleukin-2 ELISA. The number of CD34 cells increased from 1.50 to 20.01/microL (P < .002). The CD4 cell count increased from 236 to 452/microL (P < .002). The CD45RA/CD45RO ratio increased from 0.50 to 0.57 (P < .03). Mobilized CD4 cells were functionally intact. In conclusion, G-CSF induced increases in numbers of CD34 cells and CD4 cells in HIV-infected patients. Furthermore, the fraction of naive CD4 cells increased. These findings have implications for the design of immunotherapy or gene therapy protocols.