Objective To explore the efficacy of unrelated cord blood transplantation treatment of mucopolysaccharidosis Ⅰ(MPS Ⅰ). Methods A 4-year-and-2-month-old boy with MPS Ⅰ who received treatment of human leucocyte antigen-mismatched unrelated cord blood stem cell transplantation after diagnosis was identified.The pre-treatment regimen was Busulfan+ Cyclophosphamide+ Fludarabine(Bu/Cy4+ Flud). Bu with the dosage of 1.2 mg/kg, once every 6 hours, 4 days; Cy with the dosage of 50 mg/(kg·d) for 4 days and Flud with the dosage of 30 mg/(m2·d) lasted for 4 days, respectively.The day that the graft was transplanted was defined as 0 day, days before transplantation as negative days, days after transplantation as positive days.After pre-treatment, 4.60×107/kg of cord blood nucleated cells and 3.05×105/kg CD34 positive cells were transplanted into the child.The combination of Antihuman thymocyte globulin, Cyclosporin A and Mycophenolate mofetil was administrated for prophylaxis of graft versus host disease(GVHD). After transplantation, the patient was given granulocyte colony stimulating factor to promote reconstitution of hematopoiesis. Results The myeloid and platelet engraftment time was respectively 15 days and 24 days after transplantation.Short tandem repeat (STR) DNA fingerprinting showed a full donor chimerism on day 21 after transplantation, and the full donor chimerism was stable afterwards.The peripheral-blood α-L-iduronidase(IDUA) activity returned to the normal value, and the IDUA gene sequencing did not demonstrate any mutation in 83 days after transplantation.On day 12 after transplantation, pulmonary infection with pulmonary hypertension occurred.Grade-Ⅱ acute intestinal GVHD occurred on day 15, Grade-Ⅱ acute cutaneous GVHD on day 51, and chronic GVHD (cutaneous, localized) on day 180.Otherwise, the patient complicated with hemorrhagic cystitis on day 35.These complications was cured favourably.In an 18-month-follow-up, the height of the boy increased by 3 cm, and his body weight had increased by 2.4 kg.His corneas regained clear, and his hepatosplenomegaly disappeared.The glycosaminoglycan of urine was negative.The neurocognitive performance of the boy had a little improvement.The abnormalities of fingers and other skeletons had no marked change. Conclusions Unrelated cord blood transplantation for MPS Ⅰ have definited effect.It is the first case report in China on treatment of MPS Ⅰ by unrelated cord blood transplantation.The researchers have accumulated some preliminary experience for future treatment of MPS Ⅰ by unrelated cord blood transplantation. Key words: Hematopoietic stem cell transplantation; Cord blood; Mucopolysaccharidosis