Hunter syndrome or mucopolysaccharidosis II (MPS II) is a rare X-linked disease caused by a deficiency of the iduronate-2-sulphatase (12S) lysosomal enzyme, resulting in a progressive accumulation of glycosaminoglycans (GAGs). Enzyme replacement therapy (ERT) with recombinant human 12S idursulfase has been used infrequently in children < 5 years. We present the case of a 7 years and 10 months-old child, who was diagnosed with a severe form of MPS II at the age of 3 years, and who began a 36 months' treatment with idursulfase at 4 years 10 months. After 10 months, GAG urinary excretion was normal, but after just 4 months the liver and spleen had decreased in size, returning to normal limits by 36 months. Significant bone remodeling was noted after 16 months. Cardiac and neurological development, however, progressively deteriorated. The only adverse reactions were episodic inflammations of the upper and/or lower respiratory tract, but there was no otitis. Early use of ERT, presuming good treatment adherence, can significantly improve bone abnormalities.