The "Stiff person syndrome"(SPS) is a rare dysimmune chronic neurological disorder, sometimes paraneoplastic, characterized by progressive stiffness, painful persistent or spasmodic muscle contractions, mostly involving spine and lower extremities. In 60 to 90 percent of cases, non-paraneoplastic forms are associated to the presence of anti-glutamic acid decarboxylase (anti-GAD) antibodies in the cerebrospinal fluid and in the serum, while anti-amphiphysin antibodies are frequently associated to paraneoplastic types. The relevant treatment consists of three basic approaches: increase in the inhibitory processes in charge of muscle activity control, re-modulation of the immune response, removal of any associated neoplasia. Indications regarding the efficacy of high-dose intravenous immunoglobulin (IVIG) also in this dysimmune pathology are on the increase. We described an unusual case of autoimmune SPS associated with an exclusively motor left peroneal nerve neuropathy, with conduction block, treated with high-dose intravenous immunoglobulin (IVIG), oral cyclosporine, sodium valproate, baclofen and diazepam.