Glycogen storage disease type Ia (GSD‐Ia), which is characterized by impaired glucose homeostasis and chronic risk of hepatocellular adenoma (HCA), is caused by deficiencies in the endoplasmic reticulum (ER)‐associated glucose‐6‐phosphatase‐α (G6Pase‐α or G6PC) that hydrolyzes glucose‐6‐phosphate (G6P) to glucose. G6Pase‐α activity depends on the G6P transporter (G6PT) that translocates G6P from the cytoplasm into the ER lumen. The functional coupling of G6Pase‐α and G6PT maintains interprandial glucose homeostasis. We have shown previously that gene therapy mediated by AAV‐GPE, an adeno‐associated virus (AAV) vector expressing G6Pase‐α directed by the human G6PC promoter/enhancer (GPE), completely normalizes hepatic G6Pase‐α deficiency in GSD‐Ia (G6pc^−/−) mice for at least 24 weeks. However, a recent study showed that within 78 weeks of gene deletion, all mice lacking G6Pase‐α in the liver develop HCA. We now show that gene therapy mediated by AAV‐GPE maintains efficacy for at least 70‐90 weeks for mice expressing more than 3% of wild‐type hepatic G6Pase‐α activity. The treated mice displayed normal hepatic fat storage, had normal blood metabolite and glucose tolerance profiles, had reduced fasting blood insulin levels, maintained normoglycemia over a 24‐hour fast, and had no evidence of hepatic abnormalities. After a 24‐hour fast, hepatic G6PT messenger RNA levels in G6pc^−/− mice receiving gene therapy were markedly increased. Because G6PT transport is the rate‐limiting step in microsomal G6P metabolism, this may explain why the treated G6pc^−/− mice could sustain prolonged fasts. The low fasting blood insulin levels and lack of hepatic steatosis may explain the absence of HCA. Conclusion: These results confirm that AAV‐GPE–mediated gene transfer corrects hepatic G6Pase‐α deficiency in murine GSD‐Ia and prevents chronic HCA formation. (HEPATOLOGY 2012;56:1719–1729)