Mucopolysaccharidosis type III (MPS III) disease is an autosomal recessive disease, belonging to the group of mucopolysaccharidoses characterized by accumulation of heparin sulfate and characterized by severe and rapid intellectual deterioration. There are recognized four enzymatically distinct forms, with the prevalence of subtypes; type IIIA, IIIB, IIIC and IIID. The disorder is underdiagnosed (due to the generally very mild dysmorphism); it is the most frequent MPS in the Netherlands and Australia with respective prevalences of 1:53,0000 and 1:67,000. The frequency of the different subtypes varies between countries: Subtype A is more frequent in England, the Netherlands and Australia and subtype B is more frequent in Greece and Portugal, whereas types IIIC and IIID are much less common. The first symptoms appear between the ages of 2 and 6 years, with behavioral disorders (hyperkinesia, aggressiveness) and intellectual deterioration, sleep disorders and very mild dysmorphism. The neurological involvement becomes more prominent around the age of 10 years with loss of motor milestones and communication problems. Seizures often occur after the age of 10. A few cases of attenuated forms have also been reported. The neurological degradation accompanied by multiple complications requires a multidisciplinary management to allow adapted symptomatic treatment. The prognosis is poor with death occurring in most cases of type IIIA at the end of the second decade. Longer survival times (30/40 years) have been reported for the B and D subtypes. When these patients undergo surgical procedures, often intended to improve their quality of life, they have serious anaesthetic difficulties due to skeletal, neurologic and cardio-respiratory alterations.