A patient with Hunter syndrome, or mucopolysaccharidosis type II (MPS-osis II), was subjected to bone marrow transplantation (BMT), at the age of 2 9/12 years. A two-year follow-up ensued to the purpose of comparing clinical, biochemical, neuropsychologic status pre- and post-BMT. From the clinical standpoint, a complete normalization of hepatosplenomegaly was observed. In addition the skin decreased in thickness and joint mobility improved. The echocardiography showed normalization of left ventricle size. With the exception of verbal capabilities, there was no further deterioration of the neuropsychologic profile. The ultrastructural examination of the liver showed an almost total disappearance of storage material. Normal iduronate sulfatase levels in leukocytes and lymphoblasts were constantly found after BMT. A qualitative and quantitative improvement in urinary glycosaminoglycan (GAG) excretion was also found. The effectiveness of the BMT in our patient is also assessed in the context of the few cases of MPS-osis II that have been reported to date. A final evaluation of the efficacy of BMT in MPS-osis II will be possible only when a higher number of patients, diagnosed as early as possible and transplanted within the first months of life, can be followed-up for more extended periods of time.