Genetic deficiency of glucose-6-phosphatase (G6Pase) underlies glycogen storage disease type Ia (GSD-Ia, also known as von Gierke disease; MIM 232200), an autosomal recessive disorder of metabolism associated with life-threatening hypoglycemia and growth retardation. We tested whether helper-dependent adenovirus (HDAd)-mediated hepatic delivery of G6Pase would lead to prolonged survival and sustained correction of the metabolic abnormalities in G6Pase knockout (KO) mice, a model for a severe form of GSD-Ia. An HDAd vector encoding G6Pase was administered intravenously (2 or 5 x 10(12)vector particles/kg) to 2-week-old (w.o.) G6Pase-KO mice. Following HDAd vector administration survival was prolonged to a median of 7 months, in contrast to untreated affected mice that did not survive past 3 weeks of age. G6Pase levels increased more than tenfold between 3 days and 28 weeks after HDAd injection (P < 0.03). The weights of untreated 2 w.o. G6Pase-KO mice were approximately half those of their unaffected littermates, and treatment stimulated their growth to the size of wild-type mice. Severe hypoglycemia and hypercholesterolemia, which are hallmarks of GSD-Ia both in humans and in mice, were also restored to normalcy by the treatment. Glycogen accumulation in the liver was markedly reduced. The efficacy of HDAd-G6Pase treatment in reversing the physiological and biochemical abnormalities associated with GSD-Ia in affected G6Pase-KO mice justifies further preclinical evaluation in murine and canine models of GSD-Ia.