This study examines sexuality and HIV/AIDS prevention knowledge among minority ethnic male youth of Bangladesh. A cross-sectional survey was conducted through a self-administered questionnaire on 800 young males aged 15-24 years in the Chittagong Hill Tracts region in 2009. Of the respondents, almost one-third were sexually active and of them about one-quarter had had sex with multiple partners. Among sexually active youth the use rate of condoms was very low. Knowledge regarding HIV/AIDS transmission and its prevention method among the study youth was very poor, while HIV-related stigma, discrimination and misconception were at a moderate level. Logistic regression analyses revealed significantly (p less than 0.001) increased risk of sex with multiple partners for the adolescents and never-married youth. Ethnic identity, residence, sex of household head, mass-media exposure, sexual experience and wealth index appeared as the important determinants of HIV/AIDS-related knowledge. Knowledge of the ABC (Abstain from sex, Be faithful to partner, Condom use during sex) method to prevent HIV infection was significantly higher among youth with higher levels of education, who had access to mass media, and amongst the rich. The findings of the study suggest that tribal youth are prone to sexually transmitted diseases/sexually transmitted infections and HIV. Sexual and HIV/AIDS-related knowledge among youth should be increased through an information, education and communication programme. (Contains 1 figure and 9 tables.)