Background Surveillance studies among female sex workers (FSWs) in Mongolia have found no HIV but high rates of syphilis, ranging from 10.7% in 2002 to 20.8% in 2007.

Objectives To determine the current prevalence of HIV and syphilis among FSWs, and to identify syphilis risk factors.

Methods 761 FSWs were recruited by time–location sampling between 2 January and 29 March 2012 in Ulaanbaatar city, Darkhan-Uul, Orkhon, Khuvsgul and Dornod provinces in Mongolia. Participants were administered an anonymous structured survey. Blood samples were tested for HIV and syphilis. Bivariate and multivariate logistic regression analyses were performed to identify factors associated with syphilis.

Results The mean age of participants was 31 (median 30, interquartile range 24–38). HIV knowledge was modest: 41.3% correctly answered all questions. Consistent condom use with clients was reported by 49.1% of participants and exposure to HIV prevention programmes by 50.1%. Although no cases of HIV were found, syphilis prevalence was −27.8% (95% CI 24.3% to 31.7%). In multivariate analyses, variables associated with syphilis were younger age (age >30 years, OR=0.96, 95% CI 0.92 to 0.99) and occurrence of genital ulcer (OR=2.24, 95% CI 1.17 to 4.28).

Conclusions A syphilis epidemic continues to grow among FSWs in Mongolia. These women are at high risk of HIV transmission if introduced into their sexual networks. With the increase in migration of mining workers in Mongolia, introduction of HIV may be imminent. Efforts to intensify treatment and prevention programmes among FSWs are needed.