CD34 cells from human immunodeficiency virus (HIV)-infected persons have been described to be impaired in function. The effect of highly active antiretroviral treatment (HAART) on the function of CD34 cells in HIV-infected patients was examined. Numbers and function of CD34 cells from 11 HIV-infected patients were determined prior to HAART and after 2, 4, 8, and 12 weeks of therapy. The mean number of colony-forming units (cells) per milliliter (cfu/mL) was 15.0 prior to HAART vs. 109.8 in healthy controls (P < .001). During HAART, the number of cfu/mL increased to 100.3 (P < .001). This increase in cfu/mL eliminated the differences between HIV-infected patients and controls. Significant increases in numbers of CD34 cells were not detected. Of importance, the cloning efficiency of CD34 cells increased from 1.7% prior to therapy to a peak at 18.7% (P = .003). In conclusion, HAART normalized CD34 cell function in HIV-infected patients and thus might allow de novo production of T lymphocytes from progenitor cells.