Although efforts to control the heterosexual human immunodeficiency virus (HIV) epidemic in Thailand had shown success by the late 1990s (1), HIV continued to spread in other risk groups, including men who have sex with men (MSM). In 2003, the Thailand Ministry of Public Health–U.S. CDC Collaboration (TUC) started surveillance among MSM in Bangkok, finding an HIV prevalence of 17.3% (2). By 2005, HIV prevalence in this group had risen to 28.3% and has since stabilized at around 30% (2,3). To obtain additional information about HIV and sexually transmitted infection (STI) prevalence and incidence in a clinic-based population of MSM, TUC, in collaboration with the Thai Red Cross AIDS Research Center, analyzed data collected at the Silom Community Clinic (SCC), an HIV and STI testing center targeting MSM. This report describes trends in HIV and syphilis prevalence and incidence seen among SCC MSM clients during 2005–2011. At first clinic visit, the prevalence of HIV infection among 4,762 clients was 28.3% and of syphilis (all stages) was 9.8%. Among those returning for HIV or syphilis testing before the end of 2011, the incidence of HIV infection was 6.3 per 100 person-years (PY) and 3.6 per 100 PY for syphilis. These results show ongoing epidemics of HIV and syphilis infection in MSM in Bangkok, underscoring the urgent need for preventive interventions to reduce the spread of HIV and STI in this population.