Based on Italian and international literature, the authors have studied nineteenth-century research into neurosyphilis in its main syndromic manifestations: tabes dorsalis and progressive paralysis. By examining some psychiatry handbooks of the time, it was possible to recover ample information about the symptomatology and the description of the clinical course of the illness. Moreover, the relationship between syphilis and mental illness became a conceptual paradigm which psychiatrists applied to other mental disorders as well. Our paper has brought to light the medical debates of the late 19th century.