Enzyme replacement therapy (ERT) has been developed for several lysosomal storage disorders, including mucopolysaccharidosis I (MPS I), and is effective at reducing lysosomal storage in many tissues and in ameliorating clinical disease. However, intravenous ERT does not adequately treat storage disease in the central nervous system (CNS), presumably due to effects of the blood-brain barrier on enzyme distribution. To circumvent this barrier, we studied whether intrathecal (IT) recombinant human alpha-L-iduronidase (rhIDU) could penetrate and treat the brain and meninges. An initial dose-response study showed that doses of 0.46-4.14 mg of IT rhIDU successfully penetrated the brain of normal dogs and reached tissue levels 5.6 to 18.9-fold normal overall and 2.7 to 5.9-fold normal in deep brain sections lacking CSF contact. To assess the efficacy and safety in treating lysosomal storage disease, four weekly doses of approximately 1 mg of IT rhIDU were administered to MPS I-affected dogs resulting in a mean 23- and 300-fold normal levels of iduronidase in total brain and meninges, respectively. Quantitative glycosaminoglycan (GAG) analysis showed that the IT treatment reduced mean total brain GAG to normal levels and achieved a 57% reduction in meningeal GAG levels accompanied by histologic improvement in lysosomal storage in all cell types. The dogs did develop a dose-dependent immune response against the recombinant human protein and a meningeal lymphocytic/plasmacytic infiltrate. The IT route of ERT administration may be an effective way to treat the CNS disease in MPS I and could be applicable to other lysosomal storage disorders.