The purpose of this study was to assess self-reported prevalence of alcohol, tobacco, and drug use, and sexual risk behaviors, as well as to identify sources of sex education and individual, family, and peer factors that might influence adolescents’ health risk behaviors (HRB). In a community-based, cross-sectional study, 1360 adolescents aged 14–19 years were interviewed from June to September 2008 in Luang Namtha province. Multistage systematic random sampling was used to select participants. Logistic regression was used to identify the determinants of single HRB. Alcohol and tobacco use, and sexual risk behaviors were common. Just under half (42.5%) used alcohol, 9.6% smoked during the past 30 days, and 1.4% had used amphetamine during their lifetime, and 19.6% had ever had sex. Of those, 58.8% were currently sexually active, 40.1% reported having two or more sexual partners, and 34.5% had used a condom during last sexual intercourse. The mean age at sexual debut was 15.3 years for boys and 15.1 years for girls. Khamu ethnicity, not attending school, and peers’ drinking, smoking, and using drugs were significant determinants of current alcohol use, whereas being male, not attending school, Khamu ethnicity, and peers’ smoking and using drugs predicted current smoking. Being male, younger age, Akha ethnicity, having low level of education, peers’ smoking, being sexually active, and being pregnant or having made someone pregnant were significantly associated with ever having sex, whereas being male, Akha and Khamu ethnicities, peers’ smoking, drinking alcohol, and being sexually active were correlated with sexual risk behaviors. Individual and socio-demographic background factors as well as peer influence are determinants of risk behaviors. Thus, efforts should be made to keep adolescents in school, particularly until secondary school, and increase school connectedness.