A 17-year old female with “malignant” multiple sclerosis died 45 weeks after the onset of the disease. Postmortem examination confirmed the clinical diagnosis of multiple sclerosis. After 30 weeks of rapid clinical progression, the patient was treated with two courses of plasma exchange therapy without immunosuppression. This form of therapy caused a drop in cerebrospinal fluid myelin basic protein levels in one instance, but not in another. Controlled clinical trials are required to determine whether plasma exchange therapy is of value in the management of patients with multiple sclerosis, and the monitoring of cerebrospinal fluid myelin basic protein, if elevated, may be one useful marker to follow as part of these studies.