INTRODUCTION: Hunter syndrome, or mucopolysaccharidosis type II, is an inherited 
disease linked to the X chromosome that is caused by a deficit of the enzyme 
iduronate-2-sulfatase and its main symptoms affect the bones, neurological 
system and the viscera. In order to further our knowledge of its natural 
history, a registry containing data about patients' clinical histories was 
compiled. The purpose of this review is to describe how the registry works and 
to present the initial data concerning Spanish patients with Hunter syndrome 
included in it.
DEVELOPMENT: The Hunter Outcome Survey (HOS) registry is a multi-centre, 
world-wide, observational, long-term follow-up study that is open to all 
patients diagnosed with the disease. The registry includes clinical data and 
information from the complementary examinations that are commonly performed as 
usual practice while attending these patients. Its main aims are to describe the 
population of patients with the disease, to further our knowledge of its natural 
history, to keep a check on the safety and effectiveness of enzyme replacement 
therapy in patients who are candidates for such treatment and to create a 
database that makes it possible to draw up a set of guidelines for clinical 
practice.
CONCLUSIONS: Specific registries of low-prevalence diseases, such as the HOS 
registry for patients with Hunter syndrome, are important to improve the 
follow-up of patients and to determine the impact of new treatments. The Spanish 
HOS registry is an important step forward in furthering our knowledge of the 
current situation of the patients registered throughout the country.