To assess the prevalence of HIV and selected sexually transmitted diseases (STDs) among men who have sex with men (MSM) in Chengdu, China, and the risk factors associated with HIV infection. A cross-sectional study using a snowball sampling method was conducted from March to July 2007. Participants were asked to complete a questionnaire about sexual history, high-risk behaviors, STD infection history, HIV knowledge and testing, and an assessment of depression. Blood samples were taken for antibody testing for HIV, herpes simplex virus 2 (HSV-2), and syphilis. A total of 538 MSM were recruited, and 513 (95.4%) consented to complete the questionnaire. HIV, HSV-2, and syphilis prevalence were 9.1%, 24.7%, and 28.1%, respectively. The rate of consistent condom use was low and varied by types of sexual partners. The highest was with casual male partners (38.6%), and the lowest was with wife or girl friend (17.8%). Money boys were 6 times more likely to be infected with HIV compared with clerks/students. Infection with either HSV or syphilis increased the risk of HIV infection more than 4-fold. The prevalences of HIV and STDs were high among MSM in Chengdu. To prevent HIV/STDs, campaigns promoting condom use are needed not only to boost the frequency of condom use but also to educate MSM about proper condom use.