Increasing prevalence of sexually transmitted diseases (STD) was reported among men who have sex with men (MSM) in China. A total of 407 MSMs were recruited by respondent-driven sampling (RDS) from May to July 2008 in Nanjing, China. Face-to-face interviews were conducted to collect information on demographic and risk behaviors; blood samples were collected for HIV and syphilis antibodies testing. The adjusted HIV and syphilis prevalence were, respectively, 7.3% (confidence interval [CI] = 2.6%-10.2%) and 14.4% (95% CI = 9.3%-21.1%). Recruiting male sex partners mostly from saunas and receptive anal intercourse with men in the last 6 months were independently associated with positive HIV status. Risk factors for positive syphilis status included being older than 28 years, low level of HIV-related knowledge, having been a male sex worker, and having unprotected anal sex with casual male sex partners in the past 6 months. High HIV/STD prevalence was detected. HIV/STD prevention strategies are urgently warranted.