OBJECTIVE To report a case of multifocal cortical encephalitis associated with thymoma and to establish an association of this thymoma-related paraneoplastic syndrome with voltage-gated potassium channel antibodies. METHODS Case report. METHODS University hospital. METHODS A 43-year-old woman with a history of seropositive myasthenia gravis and successfully treated invasive thymoma. Four years after thymectomy, she presented with seizure and rapidly progressive confusion and aphasia. Myasthenia gravis remained in pharmacological remission. Magnetic resonance imaging of the brain showed innumerable cortically based signal abnormalities as well as extensive left mesial temporal lobe abnormality with minimal enhancement. RESULTS Chest computed tomography showed abnormal pleural thickening of the left lung, which proved to be recurrent metastatic thymoma. Results of serological evaluation were positive for acetylcholine receptor, striational, and voltage-gated potassium channel antibodies. She showed partial improvement in response to immunotherapy and chemotherapy but ultimately died 2 months later of tumor complications. CONCLUSIONS Thymoma and myasthenia gravis may be associated with other autoimmune neurological disorders including paraneoplastic encephalitis. This second case of thymoma-associated multifocal cortical encephalitis demonstrates that autoimmune encephalitis can extend to cortical regions outside the limbic system. Autoimmune encephalitis should be considered in the differential diagnosis of patients with myasthenia gravis or thymoma who develop new cognitive symptoms.