The clinical features are described in 31 cases of the mild form of Hunter's syndrome (mucopolysaccharidosis II) ascertained in the British Isles. The mean age at onset was 4.3 years and at death was 21.7 years. Each patient had a large head and short stature. Umbilical and inguinal herniae were recorded in 95% and 61% of the cases. Evidence of cardiac disease was found in 91%; this was the most common cause of death. All of the patients suffered from frequent upper or lower respiratory tract infection. Middle airways obstruction proved to be a particular hazard. A high incidence of sensorineural deafness and unexplained papilloedema was noted. The importance of regular health care for these individuals is stressed.