Mucopolysaccharidosis Type II, is a lysosomal storage disease caused by a deficient (or absent) enzyme, iduronate-2-sulfatase (I2S). The syndrome is named after physician Charles A. Hunter (1873–1955), who first described it in 1917.

The symptoms of Hunter syndrome (MPS II) are generally not apparent at birth, but usually start to become noticeable after the first year of life. Often, the first symptoms of Hunter syndrome may include abdominal hernias, ear infections, runny noses, and colds. As the buildup of GAG continues throughout the cells of the body, signs of Hunter syndrome become more visible. Physical appearances of many children with Hunter syndrome include a distinctive coarseness in their facial features, including a prominent forehead, a nose with a flattened bridge, and an enlarged tongue. The thickening of the heart valves along with the walls of the heart can result in progressive decline in cardiac function. The walls of the airway may become thickened as well, leading to breathing problems while sleeping (obstructive airway disease).

The quality of life remains high in a large number of people, and many adults are actively employed. In contrast, others with Hunter syndrome develop severe mental impairment and have life expectancies of 15 years or fewer.