A 12-year-old girl with Sly disease (mucopolysaccharidosis VII; beta-glucuronidase deficiency), who is homozygous for the A619V mutation, had a successful allogeneic BMT, donored by an HLA-identical unrelated female to replace the deficient enzyme. Within 5 months after BMT, the enzyme activity of the recipient's lymphocytes increased to normal range. No signs of acute or chronic GVHD were observed. For the successive 31 months post-BMT, beta-glucuronidase activity in her lymphocytes was maintained at almost normal levels and excretion of glycosaminoglycans in the urine was greatly diminished. Ultrastructural findings demonstrated no abnormal vacuoles and inclusion bodies in the cytoplasm of her rectal mucosal cells. Coincident with the restoration of the enzyme activity, clinical improvement was dramatic. Especially notable were improvements in motor function. The patient was able to walk alone for a long time without aid, and she even became able to ride a bicycle and take a bath. In addition, recurrent infections of the upper respiratory tract and the middle ears decreased in frequency and severity, and dyspnea on exertion, severe snoring and vertigo have substantially improved. Thus, allogeneic BMT in this patient produced a better quality of life and provided a more promising outlook.