The purpose of this study is to determine whether the development of antibodies to laronidase in patients with MPS I receiving Aldurazyme® impairs the clearance of GAG substrate.

Inclusion Criteria:

• Have a documented diagnosis of MPS I confirmed by measurable signs and symptoms and deficient α-L-iduronidase activity (<10% of the lower limit of normal).
• For a patients receiving Aldurazyme therapy prior to study entry: Have available as baseline data the results of urinary GAG levels and IgG antibody titers collected prior to the patient's first Aldurazyme infusion.
• Provide signed, written informed consent prior to any protocol-related procedures being performed. Consent of a legally authorized guardian(s) is (are) required for patients under 18 years.

Exclusion Criteria:

• Have previously received Aldurazyme without the collection of baseline samples as specified.
• Have a suspected hypersensitivity to Aldurazyme or a know hypersensitivity to components of infusion solution.
• Have received a Hematopoietic Stem Cell Transplantation, injection fibroblast therapy, or major organ transplant.
• Are receiving chronic immunosuppressant therapy.
• Have a medical condition, serious intercurrent illness, or other extenuating circumstances that may interfere with study compliance including all prescribed evaluations and follow-up activities.
• Are pregnant or lactating
• Have received investigational drug within 30 days prior to study enrollment