A 59-year-old man presents with expressive aphasia and short term memory deficits. Shortly thereafter, he started developing staring spells and intermittent right hand spasms, preliminarily thought to be simple partial seizures. Subsequent MRI brain imaging was highly suggestive of herpes simplex virus (HSV) encephalitis; however, HSV PCR from cerebrospinal fluid was negative. On further testing, the patient was found to have an autoimmune encephalitis thought to be related to an incidentally found thymoma. His clinical presentation, in conjunction with imaging and response to therapy, was strongly suggestive of thymoma associated paraneoplastic encephalitis. Early recognition is the only way to ensure prompt initiation of appropriate treatment. Immunotherapy and cancer directed therapy (including tumour resection, if indicated) have been shown to have favourable outcomes, improved speed of neurological recovery and reduced risk of relapses. Without treatment, progressive neurologic deterioration can occur over months to years, eventually resulting in death.