Bangkok, the capital of Thailand, is one of the most populous cities in the world. Many entertain- ment areas in the city tempt the young into unhealthy behavior such as drinking alcohol, drug abuse, and unprotected sex. The Bangkok Metropolis Administra- tion (BMA) has placed a high priority on HIV/AIDS prevention and control by conducting a Behavioral Surveillance Survey (BSS) since 2000 among target groups vulnerable to HIV/AIDS, including adoles- cents. The results of the BSS are used as input into programs to reduce the incidence of new cases of HIV/AIDS. The aims of this study are to determine trends in having sex, using condoms, and having multiple partners, and to explore the risk factors for sexual risk behavior among adolescents in Bangkok