A 20-month-old infant with Turner syndrome presented with opsoclonus-myoclonus and tonic pupils in association with an abdominal neuroblastoma. Despite complete removal of the tumor, the child developed progressive hearing loss, areflexia, and seizures. Immunohistochemical and Western blot studies of serum and cerebrospinal fluid revealed the presence of anti-Hu antineuronal antibody, which cross-reacted with areas of the patient's tumor. Treatment with intravenous immunoglobulin coincided with the resolution of opsoclonus-myoclonus and the cessation of new neurologic symptoms. This case provides direct support for the autoimmune basis of paraneoplastic symptoms associated with neuroblastoma and suggests that treatment with intravenous immunoglobulin may be of value.