Limbic encephalitis is one of the most common paraneoplastic neurological syndromes, which is mostly associated with small cell lung cancer, testicular tumors, and breast cancer. F-FDG PET/CT can have an important role both in the identification of limbic encephalitis itself and in the detection of the unknown malignancy that might have caused it. We report a case of a 57-year-old female patient with paraneoplastic limbic encephalitis, confirmed both by MRI and F-FDG PET/CT, and the concurrent identification of an unknown lung malignancy by whole-body PET/CT.