An 11-month-old boy with late infantile metachromatic leucodystrophy was given a bone-marrow transplant (BMT) from an HLA-identical sister; 6 months later his cerebrospinal fluid leucocytes were exclusively of donor origin. Coupled with the patient's continued developmental progress, as assessed 33 months after the procedure, the findings suggest that BMT may be an effective treatment for some congenital metabolic disorders which affect the central nervous system.