High-dose intravenous immune globulin (IVIg) has emerged as an important therapy for various neurologic diseases. Different interpretations of clinical trial results; the expected benefit of IVIg compared with that of alternate therapies; and issues about IVIg's safety, cost, and mechanisms of action have raised concern and uncertainty among practitioners. To clarify these areas, this paper examines the clinical, serologic, and immunologic data on more than 110 patients with various autoimmune neurologic diseases who received IVIg during the past 6 years at the National Institute of Neurological Disorders and Stroke. It also reviews work by other investigators on the efficacy, risks, benefits, and mechanisms of the action of IVIg in these diseases. In controlled clinical trials, IVIg has been effective in treating the Guillain-Barré syndrome, multifocal motor neuropathy, chronic inflammatory demyelinating polyneuropathy, and dermatomyositis. In other controlled or open-label trials and case reports, IVIg produced improvement in several patients with the Lambert-Eaton myasthenic syndrome and myasthenia gravis but had a variable, mild, or unsubstantiated benefit in some patients with inclusion-body myositis, paraproteinemic IgM demyelinating polyneuropathy, certain intractable childhood epilepsies, polymyositis, multiple sclerosis, optic neuritis, and the stiff-man syndrome. The primary adverse reaction was headache; aseptic meningitis, skin reactions, thromboembolic events, and renal tubular necrosis occurred rarely. The most relevant immunomodulatory actions of IVIg, operating alone or in combination, are inhibition of complement deposition, neutralization of cytokines, modulation of Fc-receptor-mediated phagocytosis, and down-regulation of autoantibody production. Therapy with IVIg is effective for certain autoimmune neurologic diseases, but its spectrum of efficacy has not been fully established. Additional controlled clinical trials are needed.