We reported a case of opsoclonus-myoclonus syndrome. A 63-year-old man was admitted to Kenwakai Hospital with rapidly progressing symptoms, including lumbago, whole body pain, vertigo, nausea, and anorexia. He became bed-ridden because of severe vertigo and truncal ataxia. Five days after admission, he developed opsoclonus followed by myoclonus and mild disturbance of consciousness, but he showed no appendicular ataxia or pyramidal tract sign. He was treated with prednisolone, 40 mg/day, which was effective for disturbance of consciousness, but opsoclonus and myoclonus persisted. He died of liver dysfunction and ventricular fibrillation 3 weeks after onset. Blood examination revealed high LDH (1,106 IU/l), Al-P, and gamma-GTP titers. Tumor markers were normal except for increase NSE activity (129 ng/ml). The cerebrospinal fluid showed normal cell count, 63.9 mg/dl of protein, 7.3 mg/dl of IgG, and normal glucose. A cranial CT scan showed an old lacune only. Chest rentgenogram and CT scan revealed mediastinal and hilar lymph node enlargement. An abdominal CT scan showed multiple low density masses in the liver. Small cell lung cancer associated with opsoclonus-myoclonus syndrome was suspected. Western blot analysis revealed that his serum reacted with protein in the cerebellum, cerebrum, and dorsal root ganglion with a molecular weight of 77 kDa. This is the first time such an antibody was ever been detected in patients with opsoclonus-myoclonus syndrome. The molecular weights of the antigens previously found by the serum of patients with this syndrome, were 55 kDa and 80 kDa in patients with breast cancer, and 210 kDa in patients with neuroblastoma.(ABSTRACT TRUNCATED AT 250 WORDS)