Neurosyphilis can present with psychiatric symptoms. The late form can occur in old age with psychosis, paranoid delusions, affective disorders or cognitive impairment. Here we present a case of neurosyphilis in an elderly woman who, over six months, progressively manifested personality changes and paranoid delusions which were initially suspected as Alzheimer's disease. Psychotic symptoms showed a good response to antibiotic treatment. We conclude that neurosyphilis is a relevant differential diagnosis in patients developing severe psychiatric symptoms in old age. As a causal antibiotic treatment is possible this infectious disease should be considered seriously in gerontopsychiatric patients.