Autoimmune synaptic encephalitis can occur as paraneoplastic neurological syndromes, which are dysfunctions of the nervous system in cancer patients. One such rare but treatable form is associated with GABAB (γ-aminobutyric acid-B) receptor antibody. We report a 55-year-old man with small cell lung cancer who presented with 3 weeks of progressive seizures, memory impairment, and behavioral disorder. His cerebrospinal fluid anti-GABAB receptor antibody titer was elevated. F-FDG PET/CT revealed pronounced medial temporal hypermetabolism with gross hypometabolism in the rest of the brain. There were no associated abnormalities on MRI. He showed improvement after immunotherapy and chemoradiotherapy.