The pathogenesis of central nervous system paraneoplastic syndromes has not been established. Increasing evidence supports the hypothesis that CNS paraneoplastic syndromes are immune disorders arising from an immune response to the underlying neoplasm. The hypothesis is that antigens or epitopes shared between the tumor and the nervous system are recognized as foreign by the host and elicit an immune response. The immune response is directed appropriately against the tumor leading to a more indolent course in those patients with paraneoplastic syndromes but is misdirected against the nervous system often leading to severe nervous system destruction. The evidence for this hypothesis consists of: 1) The finding of high titer autoantibodies that react with antigens in both tumor and central nervous system. 2) Intrathecal synthesis of the antibody. 3) The presence of the antibody in the central nervous system at autopsy. The development of animal models will be required to determine the exact nature of the pathogenesis of these disorders.