A 23-year-old woman with juvenile-onset alpha-mannosidosis developed an axonal polyneuropathy more than a year following successful unrelated donor (URD) BMT complicated by chronic graft-versus-host disease (GVHD). Progressive muscle weakness and paresthesias developed over at least 4 months, and made her nonambulatory. Nerve conduction and EMG studies demonstrated an axonal sensorimotor neuropathy. Cerebral spinal fluid (CSF) IgG was elevated with two peaks not identified in serum. Strength improved after a single course of plasma exchange and continued to improve over 12 months. The response to plasma exchange, elevated CSF IgG production, and evidence of a serum IgM peak suggest an immune-mediated mechanism. Chronic polyneuropathies following BMT are rare and are usually temporally related to GVHD or infection. This patient's disease was unusual because of its late occurrence and chronic onset in the face of resolved GVHD and in the absence of infection.