Abstract: The present invention relates to the intrathecal (IT) administration of recombinant enzyme to treat lysosomal storage disorders. In an exemplary embodiment, intrathecal administration of human ?-L-iduronidase (rhIDU) injections in MPS I affected animals resulted in significant enzyme uptake, significant rh-iduronidase activity in brain and meninges and a decrease of glycosaminoglycan (GAG) storage in cells of MPS I subjects to that of normal subjects. Intrathecal administration proved more effective than intravenous treatment at alleviating MPS I symptoms, indicating it is a useful method of treating lysosomal storage disorders.