The twitcher mouse is an animal model of galactosylceramidase deficiency (Krabbe's disease), a human sphingolipidosis. The effects of hematopoietic cell transplantation as potential enzyme replacement therapy were examined in the twitcher mouse. Survival in twitcher mice with transplants was significantly prolonged and was associated with gradual repair of demyelination in peripheral nerves. In contrast, there was no improvement in the neurodegenerative process in the central nervous system after transplantation. These observations indicate that cellular transplantation may effectively provide in vivo enzyme replacement for the peripheral manifestations of genetic storage diseases. Strategies to perturb the blood-brain barrier may be necessary for enzyme replacement to be therapeutic in diseases with central nervous system manifestations.