General paresis of the insane (GPI) remains the form of neurosyphilis most closely associated with dementia, even after the advent of penicillin. Penicillin remains the top treatment choice for syphilis, but treatment failure is not rare. Although the neurological symptoms of GPI can be alleviated by antibiotic treatment, mental symptoms may continue. A 60-year-old man was admitted to hospital due to rapidly progressive dementia. He was diagnosed as GPI. With the patient’s informed consent, we treated him with a large dose of intravenous immunoglobulin (IVIG) (0.4 g/kg/day) for 5 days, as well as penicillin (24 million units daily divided into six doses) for 14 days. A near-immediate improvement in his emotions and orientation occurred on the 17th day in hospital. The patient made an excellent recovery 6 weeks after treatment, his psychotic and mood symptoms improved significantly. Therefore, we hypothesize that patients with GPI treated with IVIG and penicillin G would have better outcomes than those treated with penicillin G alone. IVIG may be introduced as a necessary treatment for GPI.