There are no established treatments for patients with acute, severe neurological deficits caused by multiple sclerosis or other inflammatory demyelinating diseases of the central nervous system who fail to recover after treatment with high-dose corticosteroids. We conducted a randomized, sham-controlled, double-masked study of plasma exchange without concomitant immunosuppressive treatment in patients with recently acquired, severe neurological deficits resulting from attacks of inflammatory demyelinating disease, who failed to recover after treatment with intravenous corticosteroids. Patients who did not achieve moderate or greater improvement after the first treatment phase crossed over to the opposite treatment. Moderate or greater improvement in neurological disability occurred during 8 of 19 (42.1%) courses of active treatment compared with 1 of 17 (5.9%) courses of sham treatment. The primary analysis was positive. Improvement occurred early in the course of treatment, and was sustained on follow-up. However, 4 of the patients who responded to the active treatment experienced new attacks of demyelinating disease during 6 months of follow-up. Moderate or greater improvement occurred during follow-up in only 2 of 13 patients who failed to improve during the treatment phase. Plasma exchange leads to functionally important neurological recovery in an important proportion of severely disabled patients with acute attacks of idiopathic inflammatory demyelinating disease.