Adolescents are a primary smoking prevention target of the Thai government. Assisting Thai adolescents to refrain from smoking will have long‐term benefits for the adolescents and also for the society. The current study is designed as a prospective cohort survey aimed at evaluating national level tobacco control policies on the smoking behavior of both adolescents and adults. The baseline round, detailed in this report, provides estimates of smoking behaviors and attitudes among adolescents. Data was collected in January and February 2005.   Subsequent rounds of the study will be used to document changes in these behaviors over time. The cohort recruited for the study represent a nationally representative sample of adolescents (both smokers and non‐smokers) aged 13 to 17.