A 64-year-old Japanese woman presented with a three-month history of progressive numbness and weakness of the lower extremities. A neurological examination and nerve conduction study indicated sensorimotor polyneuropathy. Since the serum anti-Hu antibody titer was remarkably elevated, paraneoplastic neurological syndrome was highly suspected. A thoracoscopic biopsy of the hilar lymph nodes, in which (18)F-fluorodeoxyglucose uptake was obviously increased, revealed pathological findings for small-cell lung cancer (SCLC). Subsequently, the patient presented with generalized tonic-clonic seizures, and cerebral MRI showed reversible multifocal brain lesions, considered to reflect paraneoplastic encephalopathy. After two courses of chemotherapy for SCLC, the brain lesions totally disappeared.