The objective of this study was to assess the prevalence of not wearing a helmet (unprotected) while riding a motorcycle and associated risk behaviors among adolescents and young adults in Northern Thailand. Participants were 1725 students, aged 15-21 years, from 3 vocational schools in Chiang Rai Province; 51.8% were male. Participants completed a classroom-based computer-assisted self-interview (ACASI). Of men 72.7% and of women 64.4% reported unprotected motorcycle riding 3 times or more in the past week. Logistic regression analysis showed the variables independently associated with unprotected riding to be history of ever riding after having had 3 or more alcoholic drinks (odds ratio (OR) = 2.21, 95% confidence interval (CI) = 1.76-2.21), attending technical school (OR = 2.09, 95% Cl = 1.55-2.83), living with the family (OR = 1.38, 95% CI = 1.10-1.73), and having ever had a traffic accident (OR = 1.20, 95% CI = 1.12-1.29). Being of hill tribe ethnicity (vs Thai lowlander) was associated with protected riding (OR = 0.42, 95% CI = 0.20-0.90). Adolescents and young adults in Chiang Rai are at high risk for riding a motorcycle without a helmet buckled on the head. Public education in combination with enforcement of compulsory helmet use while riding a motorcycle is recommended.