This invention provides methods of diagnosing or determining a cause of an autoimmune encephalitis or an epilepsy in a subject and of diagnosing a tumor in a subject, comprising the step of testing a biological sample of the subject for an anti-body to a GABAB receptor. This invention further provides methods of treating an autoimmune encephalitis or an epilepsy, comprising the steps of detecting an antibody to a GABAB receptor and treating a tumor associated with the disease.
