A 66-year old male without any known psychiatric disease started to demonstrate easy irritability, poor impulse control, decreased sleep need, grandiosity and hypertalkativeness three weeks prior to his visit at our hospital. The patient neither had any physical illness nor recent conflict with his family. Mania was initially suspected when the patient was sent to our emergency department; a psychiatric consultation was accordingly requested and the patient was subsequently admitted to the acute psychiatric ward for diagnostic evaluation. Comprehensive geriatric assessment (CGA) showed mild cognitive disorder (MMSE: 26/30), no obvious depression (GDS: 0/15), and good activities of daily living (ADL: 100/100; IADL: 6/8). Elevated RPR/VDRL (1:32) was noted during his hospitalization, and active syphilis infection was confirmed by serum TPPA/TPHA (1:20480). An infectious disease specialist was consulted, and the patient was highly suspected of having neurosyphilis. CSF sampling was then performed. The lab results were compatible with diagnostic criteria for neurosyphilis. The psychotic symptoms improved gradually with intravenous ceftriaxone treatment. We also corrected newly diagnosed hypertension and chronic renal disease and gave instructions on daily diet. After discharge, his symptoms continued to improve under oral medication, and was followed up regularly until total recovery.