The present study examined the safety and relative in vivo survival of genetically engineered CD4⁺ T lymphocytes in human immunodeficiency virus (HIV)-infected individuals. Ten pairs of identical twins discordant for HIV infection were recruited, with the uninfected twin serving as the lymphocyte donor. Ten subjects were treated with a total of 19 separate infusions of retroviral vector-transduced CD4⁺ enriched T cells. Control (neo gene) or anti-HIV gene (antisense trans-activation response [TAR] element and/or trans-dominant Rev)-engineered lymphocytes were monitored in peripheral blood for 3 years, using a vector-specific PCR assay. Data from 9 of the 10 patients (15 of the 19 infusions) demonstrated preferential survival of CD4⁺ lymphocytes containing the anti-HIV gene(s) in the immediate weeks after infusion. In six of six patients studied long term (>100 weeks), only T cells containing the anti-HIV genes were consistently detected. In addition, a marked survival advantage of anti-HIV gene-containing T cells was observed in a patient treated during a period of high viral load. Thus, these data strongly support the hypothesis that anti-HIV genes afford a survival advantage to T cells and potential benefit to HIV-1⁺ individuals.