Here we provide the first evidence that therapeutic levels of a lysosomal enzyme can bypass the blood-brain barrier following intranasal administration. alpha-L-iduronidase (IDUA) activity was detected throughout the brains of IDUA-deficient mice following a single intranasal treatment with concentrated Aldurazyme(R) (laronidase) and was also detected after intranasal treatment with an adeno-associated virus (AAV) vector expressing human IDUA. These results suggest that intranasal routes of delivery may be efficacious in the treatment of lysosomal storage disorders.