Mucopolysaccharidosis type II (MPS II), also known as Hunter syndrome, is a chronic and progressive X-linked lysosomal disease that mainly affects males. It occurs in 1 in every 65,000 to 1 in 132,000 births. There are two distinct forms of the disease based on age of onset and clinical course: mild and severe. MPS II affects many organ systems including the nervous, cardiovascular, gastrointestinal and respiratory systems. Complications can include vision problems, progressive hearing loss, thickened and elastic skin, mental impairment, and enlarged liver and spleen. We herein focus on the adaptive behavior of individuals with MPS II, and the impact of MPS II on the family system. Outcomes from the Vineland-II Adaptive Behavior Scales showed that the MPS II patient sample experienced significantly lower functioning in communication, daily living skills, socialization, and motor skills compared to normative data. Patients with severe MPS II were found to have significantly lower adaptive functioning in all domains, as compared to those with mild MPS II. Length of time on ERT had no significant relationship to adaptive functioning. Results from the Peds QL Family Impact Module indicated that families of patients with MPS II experienced a lower overall health-related quality of life and overall lower family functioning (including lower emotional and cognitive functioning) than those with chronic illnesses residing in an inpatient setting.