According to prognosis, in the coming years we can expect an increase in the number of patients suffering from both HIV infection and syphilis. HIV-infected people are at high risk of neurosyphilis (NS). Diagnosing NS in case of HIV infection presents certain difficulties due to the ambiguity of the criteria for evaluating cerebrospinal fluid (CSF) indices. Objective — to assess the incidence, clinical manifestations of NS in case of HIV co-infection, the effectiveness of the applied diagnostic criteria and treatment regimens. Material and methods. The medical records of 103 HIV-positive patients with syphilis who were monitored at the St. Petersburg Center for the Prevention and Control of AIDS and Infectious Diseases and whose CSF was examined prior to treatment were reviewed. Results. Young men who had sex with men were predominant among HIV-infected patients with syphilis. The diagnosis of NS was made for 67% of HIV-infected patients, however, there was more than 2-fold overdiagnosis of specific lesions of the nervous system, due to the underestimation of the role of HIV infection in the development of the CSF pathology. A lack of correlation between the titers of microprecipitation serum test (MPST) and the duration of syphilis was identified. In HIV-infected patients with NS, the MPST titer was significantly higher than in those without damage to the nervous system (p=0.027). The sensitivity of MPT with liquor (MPLT) as an indicator of NS was 26.7%, of pleocytosis, 83.9%, of hyperproteinarhia, 77.4%. Conclusion. In HIV-infected patients with syphilis in case of negative result of MPLT and positive treponemal tests with CSF (TLT) the criteria for diagnosis of NS should be pleocytosis of more than 10 cells per 1 μl (for patients receiving antiretroviral therapy) and more than 20 cells in 1 μl (for patients who do not receive the therapy).