Summary:The opsoclonus‐myoclonus syndrome (OMS) is a potentially devastating paraneoplastic or paraviral syndrome. Although it is a rare disorder, it has major implications for cancer, virology, immunology, developmental neurobiology, and molecular pharmacology. The mechanism of brain injury in OMS is unknown, but evidence suggests immune system dysregulation. This article surveys recent clinical and laboratory evidence for the autoimmune theory and discusses how some current therapies for OMS may exert their effects through immunomodulation. Specific testable hypotheses on the immunologic defect in OMS involving both B cells and T cells, the nature and mechanisms of brain injury, and their clinical correlations are proffered. The current therapeutic armamentarium provides a broad spectrum of nonselective immunotherapies, including noncytotoxic and cytotoxic drugs, intravenous immunoglobulins, and plasma exchange, some selected for induction and others for maintenance. The use of combination immunotherapies may allow steroid sparing, targeting of more than one immunologic effector pathway, and a mixture of early‐ and late‐acting drugs. More selective immunotherapies, now available or in preclinical and clinical trials, have great potential for the treatment of OMS but require precise information on the underlying immunological problem. These data provide possible new directions for immunologic research and therapy in OMS.