The devastating consequences of mucopolysaccharidosis II (MPS II), also called Hunter syndrome, may soon be preventable. That is, if the first results of investigational enzyme replacement therapy are confirmed in randomized trials. This syndrome is caused by a deficiency of the enzyme iduronate 2-sulphatase, which is needed to break down glycosaminoglycans. Enzyme replacement therapy with idursulfase (Elaprase) was approved in the USA in 2006 and has been available in Slovakia since 2008. This treatment has a beneficial effect on some somatic symptoms and slows down the progression of organ changes. However, the treatment does not affect cognitive function or completely halt the progression of the disease. Genetic counselling is recommended for families with an affected child.