Background and aims: Hunter syndrome (mucopolysaccharidosis type II, MPS II) is an X-linked disorder of glycosaminoglycans (GAGs) catabolism caused by a deficient activity of the lysosomal enzyme, iduronate-2-sulfatase. Methods: The Hunter Outcome Survey (HOS), a global database used to characterize the natural history of the disease as well as evaluate the safety and effectiveness of enzyme replacement therapy (ERT) with idursulfase (Elaprase®, Shire HGT, Cambridge, MA, USA), was queried. Results: As of 22 January 2010, a total of 57 untreated European patients were less than 10 years old at HOS entry. In this population, the median age at the onset of symptoms and the diagnosis of Hunter syndrome was 1.5 years (10th to 90th percentile: 0.3 to 2.5 years) and 2.5 years (1.3 to 5.4 years), respectively. In patients with available data the most common clinical manifestations were: coarse facial features (median age at onset 1.8 years, prevalence, 97.6%), hepatosplenomegaly (2.2 years, 91.5%), chest and lung symptoms (1.8 years, 90.2%), enlarged tongue (2.0 years, 79.5%), enlarged tonsils/adenoids (2.8 years, 77.1%), joint stiffness (2.5 years, 76.7%), hernia (1.3 years, 74.5%), and recurrent otitis media (1.8 years, 65.9%). Neurological manifestations were reported by 95.7% of patients and included behaviour problems (3.6 years, 66.0%), hyperactivity (3.0 years, 61.7%), cognitive problems (2.9 years, 61.7%), and fine motor skills impairment (3.6 years, 48.9%). Conclusions: These results indicate the early onset of the common clinical manifestations of Hunter syndrome. These data should increase awareness and help physicians identify and diagnose Hunter syndrome at an early age.