Allogeneic bone marrow transplantation was performed on twins with Sanfilippo B disease. They were the first two patients with this disorder to undergo the procedure. There was definite evidence of engraftment as shown by conversion to donor blood group antigen and tissue type, and increased leukocyte alpha-glucosaminidase activity. Nine years post transplant, neither twin is as handicapped as her untreated brothers were at the same age, although in one twin hyperactivity and behavioural problems, characteristic of the disorder, are present. Details of the twins' intellectual development and growth, their alpha-glucosaminidase activity and urinary glycosaminoglycan excretion are reported.