Hunter syndrome (mucopolysaccharidosis II, or MPS II) is an X-linked recessive lysosomal storage disease caused by a deficiency in iduronate-2-sulfatase that leads to an accumulation of heparan sulfate and dermatan sulfate in various organs. It is a rare disorder, with an estimated prevalence of 1 in 170 000 live births. There is a higher incidence in the Ashkenazi and Sephardic Jewish populations. Hunter syndrome predominantly affects males but has been described in females, often secondary to skewed X chromosome inactivation.

There is a broad phenotypic spectrum of severity. The severe form is most common, with symptom onset typically between 2 and 4 years of age. Signs and symptoms include distinct, coarse facial features, intellectual disability, developmental delay, hepatosplenomegaly, skeletal deformities, joint stiffness, carpal tunnel syndrome, vision changes, recurrent ear infections, and progressive hearing loss. Hydrocephalus or spinal cord compression may develop. Patients typically develop valvular heart disease and obstructive airway disease.

Average life expectancy is about 10-15 years. The attenuated form of Hunter syndrome may have minimal neurologic symptoms, and these patients often have normal intelligence.