OBJECTIVES: To investigate risk factors related to HIV infection and the prevalence of selected infections, especially sexually transmitted infections (STI) among men who have sex with men (MSM). DESIGN: A cross-sectional study including questionnaire and blood sample collection. METHODS: Between January 2005 and December 2006, 753 MSM were recruited through the voluntary counselling and testing (VCT) clinic at the Chaoyang District Centers for Disease Control and Prevention in Beijing. Sera samples were collected and tested for HIV, syphilis, Chlamydia trachomatis, Ureaplasma urealyticum, Toxoplasma gondii, and hepatitis C virus (HCV). Statistical analyses were performed to identify factors related to HIV and STI, as well as other co-infections. RESULTS: The prevalence of HIV-1 infection among MSM attending the VCT clinic was 2.1%. Among all MSM, 7.0% had syphilis, 5.6% Chlamydia, 4.4% U. urealyticum, 2.5% T. gondii and 0.8% HCV (13.8% had one or more). Among HIV-negative MSM, 13.3% had one of these infections, whereas among HIV-positive MSM, 93.8% had serological evidence of one or more co-infections (P < 0.0001). Individuals with HIV infection were significantly more likely to have had more male sex partners or to report frequent receptive anal sex in the past 6 months. CONCLUSION: Serological evidence of infection with any of five STI or HIV-relevant conditions was far higher in HIV-infected than HIV-uninfected MSM. Co-infection in HIV-positive men was most frequent with syphilis. Our data suggest that strategies for HIV/AIDS and STI prevention and control among MSM should be synchronized.