High HIV prevalence and incidence burdens have been reported in men who have sex with men(MSM)in Chongqing,China.We aimed to estimate the prevalence of HIV and other sexually transmitted infections(STIs),to appraise the knowledge and risk behaviors related to HIV/AIDS among MSM,and to analyze the possible causes of deviation between behavior and knowledge to make better strategies.We recruited 617 MSM from February to July in 2008 by using a respondent-driven sampling(RDS) method in Chongqing,China.Through the collection of questionnaire-based data and biological testing results from all objects,we launched a crosssectional survey.STATA/SE was used for data analysis by frequency,ANOVA,rank sum test and logistic regression models.MSM with syphilis(OR=4.16,95%CI:2.35-7.33,P<0.0001) were more likely to be HIV infected.Being a company employee(OR=3.64,95%CI:1.22-10.08,P<0.0001) and having bought male for sex(OR=3.52,95%CI:1.10-11.32,P<0.034) were associated with a higher probability of syphilis.MSM with younger age,higher education and greater monthly income had a higher mean knowledge score.MSM who had HIV testing had a higher mean knowledge score than those who never had.Students,venues for finding sex partners by Internet and homosexuals in MSM had a higher mean knowledge score compared to other occupations,venues for finding sex partners and sexual orientation.There is an urgent need for delivery of barrier and biomedical interventions with coordinated behavioral and structural strategies to improve the effect of HIV interventions among MSM. High HIV prevalence and incidence burdens have been reported in men who have sex with men (MSM) in Chongqing, China. We aimed to estimate the prevalence of HIV and other sexually transmitted infections (STIs), to appraise the knowledge and risk behaviors related to HIV / AIDS among MSM, and to analyze the possible causes of deviation between behavior and knowledge to make better strategies. We recruited 617 MSM from February to July in 2008 by using a respondent-driven sampling (RDS) method in Chongqing, China.Through the collection of questionnaire-based data and biological testing results from all objects, we launched a crosssectional survey. STATA / SE was used for data analysis by frequency, ANOVA, rank sum test and logistic regression models. MSM with syphilis (OR = 4.16, 95% CI: 2.35-7.33, P <0.0001) were more likely to be HIV infected. Having a company employee (OR = 3.64,95% CI: 1.22-10.08, P <0.0001) 3.52, 95% CI: 1.10-11.32, P <0.034) were associated with a higher probability of syp hilis.MSM with younger age, higher education and greater monthly income had a higher mean knowledge score. MS who had HIV testing had a higher mean knowledge score than those who never had. Students, venues for finding sex partners by Internet and homosexuals in MSM had a higher mean knowledge score compared to other occupations, venues for finding sex partners and sexual orientation. There is an urgent need for delivery of barrier and biomedical interventions with coordinated behavioral and structural strategies to improve the effect of HIV interventions among MSM.