Purpose:Hunter syndrome (Mucopolysaccharidosis II) is a rare, X-linked disorder of glycosaminoglycan metabo-lism. It is caused by a deficiency in the lysosomal enzyme iduronate-2-sulfatase, and in affected patients glycosaminoglycan accumulates in lysosomes of various tissues and organs and contributes to the pathophysiol-ogy of Hunter syndrome. The Hunter Outcome Survey (HOS) was established to better describe the natural history of this disorder and to evaluate the long-term effect of enzyme replacement therapy.Methods:HOS is an international, multicenter, long-term observational survey that will collect data on participating patients with a confirmed diagnosis of Hunter syndrome. Data will be collected during regular physician examinations and entered into an electronic database. Examples of observations include vital signs, laboratory values, signs and symptoms of organ involvement, and the results of selected functional tests (e.g., audiometry, echocardiogram, joint mobility, etc.).Results:As of May 15, 2007, 263 patients from 16 countries have enrolled in HOS; 24% of these patients were currently being treated with enzyme replacement therapy. The median age at enrollment was 12.2 years. The median age of onset of symptoms and diagnosis of Hunter syndrome were 1.5 and 3.5 years, respectively. Otitis media and abdominal hernia were the earliest presenting symptoms. Facial dysmorphismand hepatosplenomegaly were demonstrated by 95% and 89% of patients, respectively.Conclusions:HOS will be a valuable resource for enhancing the understanding of Hunter syndrome and will provide important information about the natural history of the disease and the role of enzyme replacement therapy in its treatment. Patients and their physicians should be encouraged to participate.