Two patients with hematologic malignancy and quiescent inflammatory demyelinating polyneuropathy developed severe exacerbations of polyneuropathy at the time of bone marrow transplantation. The clinical course in both patients was progressive despite therapy with immuno-suppressive agents, plasmapheresis, and high dose immunoglobulin. The polyneuropathy resulted in quadriplegia which contributed to the patients' deaths 175 and 48 days after transplantation. Sections of multiple peripheral nerves sampled post mortem in the first case revealed prominent demyelination with heavy infiltration of macrophages and lymphocytes. Immunohistochemical studies demonstrated that most of the lymphocytes were of the CD8+, cytotoxic/suppressor cell class and that many of the Schwann cells expressed class II (HLA-DR) antigen. This report suggests that bone marrow transplantation can exacerbate inflammatory demyelinating polyneuropathy.