Paraneoplastic syndromes affecting the nervous system are rare neurologic syndromes caused by cancer but not ascribable to metastases. Any portion of the nervous system may be involved in a paraneoplastic syndrome. The pathogenesis of these disorders appears to be an immune reaction against antigens shared by the cancer and the nervous system. Some disorders, such as the Lambert-Eaton myasthenic syndrome, are effectively treated by removal of autoantibodies directed against the presynaptic cholinergic synapse. In other disorders, such as paraneoplastic cerebellar degeneration or paraneoplastic sensory neuronopathy, neither removal of the autoantibody nor treatment of the cancer is effective.