China HIV surveillance data have shown that heroin users and sex workers in China are rapidly spreading HIV infection to the general population, but so far homosexual transmission has been ignored. There are an estimated 30 million men who have sex with men (MSM) in China, a population that can be extremely vulnerable to HIV/AIDS depending on specific risk behaviors. Currently, information is lacking about the prevalence of HIV, its incidence and which social, behavioral and contextual factors influencing the infection in the MSM population. About 70% of gay men are married or will marry under social or family pressure, thus raising the possibility of spread of infection from infected men to their spouses or children. Because most MSM tend to keep their homosexuality secret, no well-designed epidemiological studies have been conducted to determine the prevalence and incidence of HIV infection and its associated factors in this population. This R21 application proposes to conduct an epidemiologic study that will recruit 750 MSM aged 18 to 40 years old, in 5 cities in China. The specific aim of this study is to assess the feasibility of, and methods for recruitment and retention of MSM, and to estimate the prevalence and incidence of HIV infection and sexually transmitted diseases in MSM in 5 cities in China. A 2-year study is proposed to achieve this aim. This project may be considerably challenging, but it could lead to a breakthrough in HIVrelated research in China. The study could have major impact on a HIV-related biological, behavioral, and clinical research that extends beyond China to include all populations at risk for HIV infection.