Effective treatment of PNS requires an early clinical suspicion followed by rapid diagnosis, through detection of onconeural antibodies, and identification of the underlying tumor. Unfortunately appropriate treatment of the cancer is most often unsuccessful to improve neurological symptoms and these patients are generally left bedridden until death. It's seems important to associate an immune treatment but has never been adequately tested. Experience with immunotherapy modalities, such as corticosteroids, plasma exchange, immunosuppressants or human intravenous immunoglobulin (IVIg), relies on case reports, retrospective studies, and a couple of prospective studies in patients generally treated at a late stage. To date, it has not been possible to set up a prospective therapeutic trial evaluating the role of early immune treatment, administered less than 6 months after the onset of symptoms, in these patients. Considerable evidence for an initial inflammatory stage (early in the evolution of the PNS), and isolated case reports of early immunological intervention suggest that this approach may be valid to treat these disorders. Because IVIg are often use in neuro-immunological disorders (ex. first-line therapy in Guillain-Barre syndrome, chronic inflammatory demyelinating polyradiculoneuropathy, multifocal motor neuropathy and dermatomyositis) with little and well known side effects and no interaction with cancer treatment we decide to use IVIg for our trial.