Discussion. The epidemic of HIV/STD among MSM is severe, and the prevalence of risk sexual behaviors was at a high level among MSM in China. The risk factors for HIV/STD and factors associated with DAI reported in this study give some insights towards designing relevant prevention programs. The simplicity and feasibility of our effective intervention, enhanced VCT, makes it possible for this to be incorporated into standard VCT procedures. Further translational research is needed to investigate its effectiveness in the real-world setting. / Introduction. Men who have sex with men (MSM) have become the main group for HIV transmission in China in 2009. There have been many epidemiological cross-sectional studies targeting MSM in China in recent years. These provide limited data compared with cohort studies, which can describe the rate at which HIV/STD are spreading in a target population. Moreover, there is a dearth of intervention studies which are theory-based with rigorous research methodology in China. Last but not least, research is needed on sexual behaviors and their determinants, including cognitions from health behavioral theories, event-specific factors, etc. / Objectives. This study investigated the prevalence and incidence of HIV/STD, as well as their associated risk factors among MSM in Nanjing, China. It also explored the prevalence of unprotected anal sex (UAI) and the associated factors, including cognitions from health behavioral theories and event-specific factors. In addition, this is also one of the first studies to examine the efficacy of a randomized control trial (RCT) designed intervention, to use enhanced voluntary counseling and testing (VCT) to reduce UAI among MSM in China. / Results. The RDS-adjusted HIV, syphilis and HCV prevalence were respectively 7.3%, 14.4% and 0.2% at baseline. Of the 397 MSM who were found to be HIV seronegative at baseline, 286 (72.0%) retested at Month 6. HIV, syphilis and HCV incidence were respectively 5.12, 7.58 and 0 per 100 person-years (PY). Recruiting male sex partners mostly at saunas was the risk factor associated with being HIV seropositive at baseline (OR=3.84) and undergoing HIV/syphilis seroconversion at Month 6 (RR=2.351RR=6.72). In the RCT study, participants in the Intervention Group reported significantly less risk than those in the Control Group (UAI with any male sex partners: 48.4% vs. 66.7%;UAI with regular male sex partners: 52.2% vs. 68.9%) at Month 6. Furthermore, Perceived Behavior Control showed a significant association with DAI with both regular (AOR=0.42) and casual partners (AOR=O.73). / Subjects and Methods. A cohort study and randomized control trial were conducted. Out of 416 MSM approached by respondent driven sampling (RDS), 397 HIV negative participants were recruited to the HIV/syphilis/HCV cohort, and they were invited to return for a follow-up visit at Month 6. A subsample from the baseline cohort consisting of 307 MSM, was randomly assigned to either the Intervention Group (enhanced VCT) or the Control Group (standard VCT). Evaluation was conducted at Month 6. Both baseline and Month 6 visits consisted of VCT service and interviewing. Statistical methods such as Chi-square test, logistic regression and Poisson regression were used in this study. / Hao, Chun. / Adviser: Joseph TF Lan. / Source: Dissertation Abstracts International, Volume: 73-04, Section: B, page: . / Thesis (Ph.D.)--Chinese University of Hong Kong, 2011. / Includes bibliographical references (leaves 166-179). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [201-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract and appendixes I-II also in Chinese.