Syphilis has become a serious issue for human immunodeficiency virus (HIV)-infected patients worldwide in recent years; however, the studies related to HIV coinfection and syphilis reinfections in Istanbul, Turkey, are limited. Our objective was to determine the seroprevalence of syphilis among HIV-infected men in the city which has one of the highest HIV prevalence rates in Turkey. Two hundred and forty four (244) HIV-positive men were evaluated at Istanbul Medical Faculty, Department of Medical Microbiology from March to June 2018. Serum samples were screened for the presence of antibodies against Treponema pallidum using the chemilumines-cent microparticle immunoassay (CMIA). Samples found to be positive were investigated with the rapid plasma reagin (RPR) test and the T. pallidum hemagglutination assay (TPHA). The patients completed a questionnaire for sociodemographic data. The mean age was found to be 41.8 years; 35.6% were men who have sex with men (MSM). The overall seroprevalence of syphilis among the patients was 19.3%. MSM had a significantly higher seroprevalence than heterosexual patients (28.7%). In Turkey, there is a high seroprevalence of syphilis in HIV-infected patients, MSM being the most affected group. Therefore, H1V-infected patients should be screened for syphilis at least annually and should be informed about sexually transmitted diseases (STDs). (C) 2020 Asociaci6n Argentina de Microbiologia. Published by Elsevier Espana, S.L.U.