A patient with chronic myeloid leukemia developed Guillain-Barré syndrome 4 months after allogeneic bone marrow transplantation. Clinical improvement occurred after a series of plasma exchanges. A relapse of the Guillain-Barré syndrome was also successfully treated with plasma exchange, with an eventual near-complete recovery. Reactivation of cytomegalovirus infection as manifested by antigenemia 2 months prior to the weakness was probably the precipitating event in this patient. We advocate early treatment of post-transplant Guillain-Barré syndrome with plasma exchange.