Lentiviral vectors (LV) offer improved therapeutic benefit of gene therapy applications as a result of their ability to stably transduce a wide range of cycling and non-cycling cells. The first Phase I clinical trial using a lentiviral vector, VRX496, was initiated in January of 2003. The vector evaluated is an HIV-1 based lentiviral vector, fully gutted but retaining the 5′ and 3′ long terminal repeats, the packaging sequence, cPPT/CTS, splice donor and splice acceptor, and rev response element (RRE). VRX496 contains a splice-independent 937-base antisense sequence targeting the envelope gene in the HIV genome, and a small 186-base tag derived from the GFP gene to serve as a molecular marker for vector in HIV infected patient cells. Five (5) HIV-infected patients with CD4 counts between 200 and 500, and viral loads above 5000 copies/ml, no history of opportunistic infections, who have failed two regiments of highly active antiretroviral therapy (HAART), were serially enrolled in the trial. After leukopheresis, patient’s CD4 T lymphocytes were isolated by negative selection of CD8+ cells, and then cultured for three days in the presence of immobilized CD3/28 beads, and VRX496. Thereafter, the vector and beads were removed, cells were expanded to the dose of 1 x 1010, and then given intravenously over 15 minutes. At the time of this abstract, 4 patients have been dosed and have passed the initial 21-day safety assessment; a fifth patient is scheduled for dosing. No serious adverse events have occurred. All patients have steady CD4 counts, and viral loads have decreased below baseline in all patients, notably from 218,000 pre dose to 36,000 1 year post dosing in the first patient, and from 22,000 pre dose to 1,625 at 9 months post dosing in the second patient; patients 3 and 4 are at 3 months and 21 days post-infusion, respectively. Importantly, persistence of vector-modified CD4 cells are detected in the peripheral blood out to 9 months post dosing. This trial establishes for the first time the safety of lentiviral vectors for clinical gene therapy application.