A 13-year-old girl with the severe form of the Maroteaux-Lamy syndrome (mucopolysaccharidosis Type VI, arylsulfatase B deficiency) has had successful reconstitution with bone marrow from her HLA-MLC-matched sister who had normal arylsulfatase B activity. Full engraftment has been present for 24 months. The following biochemical and clinical changes have occurred: arylsulfatase B activity in peripheral lymphocytes and granulocytes increased to normal levels, and the activity in serial liver-biopsy specimens increased from about 3 per cent of the mean normal level 43 days after transplantation to about 16 per cent at 600 days. Urinary excretion of acid mucopolysaccharide decreased. Ultrastructural evidence of accumulated dermatan sulfate was no longer detectable in bone-marrow cells; in peripheral-blood lymphocytes, granulocytes, or platelets; or in Ito cells of liver. Twenty-four months after engraftment, hepatosplenomegaly was substantially decreased and cardiopulmonary function was normal. Visual acuity and joint mobility were also improved. The patient returned to school and continued to perform well in academic studies. Thus, bone-marrow transplantation provided a source of enzymatically normal cells, which have altered the metabolic and clinical course of the disease.