Although the incidence of neurosyphilis has declined dramatically after the World War II because of the development of antibiotics, it is recently increasing a little bit. There has been few reports on the neuroimaging of general paresis. We studied a fresh case of general paresis by X-ray CT (XCT), MRI and PET scans, and report the changes of neuroimages before and after the therapy. A 38-year-old man was admitted with disorientation, thought disorder and personality change. He also had insomnia and megalomania. There were neither obvious neurological signs nor particular changes of the XCT scan abnormalities on admission. His clinical symptoms were gradually worsened within three months, when the XCT and MRI scans showed moderate brain atrophy especially in the frontal and temporal lobes with mild dilatation of the lateral ventricles. In the PET study, the cerebral blood flow was decreased in the entire brain, especially in the frontal lobes, although the thalamus and the basal ganglia had normal levels. By immunological procedures and the cerebrospinal fluid (CSF) studies, the diagnosis of general paresis was made. He was treated with penicillin G and other antibiotics. In one year after his first therapy, many symptoms vanished. Although the cerebral atrophy was a little advanced, the cerebral blood flow was dramatically increased. As far as we know, this is the first PET study of general paresis before and after penicillin treatments.