Ideally, somatic gene therapy should result in lifetime reversal of genetic deficiencies. However, to date, phenotypic correction of monogenic hyperlipidemia in mouse models by in vivo gene therapy has been short-lived and associated with substantial toxicity. We have developed a helper-dependent adenoviral vector (HD-Ad) containing the apolipoprotein (apo) E gene. A single i.v. injection of this vector completely and stably corrected the hypercholesterolemia in apoE-deficient mice, an effect that lasted the natural lifespan of the mice. At 2.5 years, control aorta was covered 100% by atherosclerotic lesion, whereas aorta of treated mice was essentially lesion-free. There was negligible toxicity associated with the treatment. We also developed a method for repeated HD-Ad vector administration that could be applied to organisms, e.g., humans, with life spans longer than 2–3 years. These studies indicate that HD-Ad is a promising system for liver-directed gene therapy of metabolic diseases.