Introduction. According to the outpatient visit data at the Moscow Scientific and Practical Center of Dermatovenereology and Cosmetology (MSPCDC) and AIDS center in Moscow, the number of patients with syphilis/HIV co-infection has increased significantly (4.2—4.8 times) from 2012 to 2022. This indicates the need to analyze the social and behavioral characteristics of this population of patients with a combination of two severe infections. Objective. To study the socio-demographic and behavioral characteristics of patients with syphilis/HIV co-infection in comparison with similar characteristics of patients with syphilis and patients with HIV infection (comparison groups 1 and 2). Results. The comparison has shown that patients with syphilis/HIV co-infection had a higher level of education and fuller employment (differences are statistically significant). Sexual orientation of co-infected persons was mostly homosexual (63.3%), bisexual in 16.5% of cases. The proportion of co-infected patients who had sexual intercourses with accidental partners was significantly higher than similar indicators in the comparison groups. At the same time, patients with syphilis/HIV co-infection were significantly less likely to use individual protection means against STIs than those interviewed in the comparison groups. Conclusion. The social and behavioral characteristics of patients with syphilis/HIV co-infection should be considered by the attending physicians when clarifying the past medical history and examining sexual and domestic contacts of the patient. © 2025, Media Sphera Publishing Group. All rights reserved.