Stiff-man Syndrome (SMS) is a chronic, disabling neurological disorder characterized by severe and painful axial and limb rigidity enhanced by anxiety, sudden motion or external stimuli. Although the cause of SMS is unknown, immunologic mechanisms have been implicated on the basis of circulating autoantibodies in the patient's serum and CSF, against GAD (glutamic acid decarboxylase), the enzyme involved in the synthesis of GABA (gamma aminobutyric acid). Uncontrolled studies have also shown that plasmapheresis, corticosteroids and high dose intravenous immunoglobulin (IVIg) are variably effective in improving the clinical symptoms of these patients. The purpose of the present study is to demonstrate in a double blind, placebo-control design, the efficacy of IVIg in patients with SMS. The effect of IVIg will be assessed with a series of objective measurements including muscle function, mobility and stiffness. Changes in the circulating anti-GAD antibodies will be also examined and their pathogenetic role in the cause of SMS will be determined. If IVIg proves effective, it will be a valuable tool in the treatment of these patients who are currently dependent on high doses of Valium (up to 60-100 mg daily), or steroids and experience significant side effects.