AIM: Our study aim is the evaluation of long-term effects of hematopoietic stem cell transplantation on Italian patients with severe Hunter syndrome. METHODS: Four boys, suffering from Hunter syndrome, severe phenotype, received hematopoietic stem cell transplantation between 2 years 6 months and 2 years 11 months of age, from 1992 to 2001. A complete multidisciplinary evaluation of hematopoietic stem cell transplantation long-term effects was performed periodically. RESULTS: All patients achieved successful engraftment. Urine glycosaminoglycans excretion was reduced or normalized, and the activity of leukocyte iduronate-2-sulphatase enzyme, absent before hematopoietic stem cell transplantation, remained constant, in all patients. Dysostosis multiplex progressed over time, according to the natural evolution of the disease. Joint stiffness improved in all affected districts. Hepatosplenomegaly decreased until it disappeared. The cardiovascular involvement stayed unchanged, as well as hearing loss. Skin became hyperelastical; face features seemed less coarse if compared to the natural evolution of the disease. Cerebral white matter alterations were constant in time. On the contrary, the hematopoietic stem cell transplantation did not prove to have long-term effectiveness on neurological symptoms of Hunter syndrome. CONCLUSION: The hematopoietic stem cell transplantation was successful in slowing the progression of Hunter syndrome, and even the evolution of neurological feature of the disease was slower in the first years after this treatment.