Objectives : To investigate the sexual behavioral characteristics and HIV/AIDS knowledge among men who have sex with men(MSM), one of the HIV high risk groups. Methods : A three month survey among individuals who were able to be contacted was carried out over the entire Republic of Korea, between May and August, 2001. 348 individuals completed a self-administered question-naire. The data collected included demographic informa-tion, sexual behavior and AIDS knowledge. Results : Eighty-seven and ninety-two per cent of the 348 MSM were aged 20-39 years and had never been married, respectively. Fifty-five per cent of participants reported at least one sexual contact with women, and a quarter of the MSM surveyed had engaged in high-risk sexual behavior (more than 6 partners) during the previous year. About twenty per cent of the MSM had anal sex as their favorite way of having sex, and seventy-four per cent did not use condoms regularly due to loss of enjoyment, and were more likely to be engaged in risky behaviors. Only ten per cent had a regular HIV test history, and most had obtained knowledge or information on HIV/AIDS through the mass media. Conclusions : A large proportion of the MSM in Korea still remain at an elevated risk for contracting HIV infection. Change in high-risk sexual behaviors will prevent the spread of HIV infection among the MSM population, which requires public health education for preventive interventions, and should be culturally and socially specific in order to be effective.