Limbic encephalitis is a subacute onset syndrome characterized by short-term memory impairment, confusion, temporal lobe epilepsy, hypothalamic dysfunction, and psychiatric symptoms. Common causes are autoimmune disorders or idiopathic disease, although it has been associated to tumors. We report the case of a woman arrived at the emergency department having had fluctuating impaired memory for 2 weeks. The first MRI was normal. It was decided to perform lumbar puncture and PET/CT to exclude viral origin or paraneoplastic syndrome. Typical abnormalities were found in the second MRI (1 month after previous); however, FDG PET is a useful tool that contributes to early detection.