OBJECTIVE: To evaluate the effect of bone marrow transplantation in children with Hunter syndrome. STUDY DESIGN: Eight boys received a bone marrow graft between the ages of 3 and 16 years from 1990 to 2000. In 6 cases, the donor was a sibling with identical HLA status, in 1 case an unrelated donor with HLA-compatible, and in 1 case a mismatched unrelated donor. A complete multidisciplinary evaluation was performed yearly. RESULTS: Successful engraftment was achieved in all patients, with the proportion of donor cells reaching > or =95% 1 month after transplantation in all patients. Patients have been followed from between 7 and 17 years and all are still alive, except for 1 boy who died at the age of 10 from unrelated causes. Cardiovascular abnormalities stabilized in all patients, hepatosplenomegaly resolved, and joint stiffness improved, Perceptual hearing defects remained stable, and transmission hearing defects improved. Only 1 child required subsequent surgery to correct kyphosis. Neuropsychological outcome was variable and appeared to be related to the severity of the syndrome. CONCLUSIONS: Bone marrow transplantation is effective on the non neuropsychological symptoms of Hunter disease.