There is strong circumstantial evidence that multiple sclerosis (MS) is an autoimmune disease. Nonspecific immunosuppressive therapy has not been successful in altering the natural course of the illness. Bone marrow transplantation has heretofore been a radical therapy used in patients with life-threatening malignancies but has potential as a treatment for human autoimmunity. In MS there have been no controlled studies. We report here four patients with MS undergoing bone marrow transplantation with 6-48 months of follow-up. In three this was carried out for co-existing malignancy and in one as an experimental treatment for MS using the patient's unaffected identical twin as a donor. The limited outcome that can be evaluated in these patients supports further experimentation into this treatment modality in MS patients with poor prognostic indications.