A deficiency of β-glucuronidase (GUSB) causes the multisystem progressive degenerative syndrome, mucopolysaccharidosis (MPS) type VII (Sly disease)¹, which includes mental retardation². Animal homologues of MPS VII (ref. 3, 4) are models for testing somatic gene transfer approaches to treat the central nervous system in this and other lysosomal storage disorders⁵. Previous attempts to correct murine MPS VII by gene therapy have successfully treated lesions in some organs but not in the brain^6–8. Other experimental modalities have forestalled some disease progression in the brain, but only if done at birth^9,10, before the onset of severe lesions, when the animals are pheno-typically normal. We tested whether therapeutic amounts of GUSB could be delivered to the diseased adult brain by transplanting cells engineered to super-secrete the normal enzyme for export to surrounding neural tissues. Lysosomal distention was cleared from neurons and glial cells in the vicinity of the grafts, showing that the secreted enzyme could reach the diseased cells and reverse lesions in the severely diseased brain. The ability to correct established lesions will be important for the treatment of many lysosomal storage diseases affecting the brain, because most patients are not diagnosed until lesions are advanced enough to affect phenotype or developmental milestones in early childhood, and some forms of the diseases do not become apparent until later in life¹¹.