Metachromatic leukodystrophy (MLD) is a lysosomal storage disorder caused by a deficiency of arylsulfatase A (ASA) and is characterized by deposition of sulfatide in all organs, particularly the nervous system. Recently, formylglycine-generating enzyme (FGE) was found to be essential for activation of sulfatases. This study examined the utility of FGE co-expression in AAV type 1 vector (AAV1)-mediated gene therapy of ASA knockout (MLD) mice. AAV1-ASA alone or AAV1-ASA and AAV1-FGE were co-injected into a single site of the hippocampus. Enzyme assay and immunohistochemical analysis showed that ASA was detected not only in the injected hemisphere but also in the non-injected hemisphere by 7 months after injection. Level of ASA activity and extent of ASA distribution were significantly enhanced by co-introduction of AAV1-FGE. Marked reductions in sulfatide levels were observed throughout the entire brain. The unexpectedly widespread distribution of ASA may be due to a combination of diffusion in extracellular spaces, transport through axons, and circulation in cerebrospinal fluid. The rotarod test revealed improvement of neurological functions. These results demonstrate that direct injection of AAV1 vectors expressing ASA and FGE represents a highly promising approach with significant implications for the development of clinical protocols for MLD gene therapy.