PURPOSE:To examine the trends of substance use and correlated variables in high-school students in Southern Thailand. METHODS:Surveys of high-school years 7, 9 and 11 and vocational school year 2 students in four provinces in Southern Thailand were conducted in 2002-2004 to examine lifetime substance use, use within 1 year and 30 days before the interview, using a self-completed questionnaire. RESULTS:The prevalence of lifetime use of any illicit substance was 5-7% overall (about 7%, 9% and 13% in boys and 2%, 1% and 3% in girls in 2002, 2003 and 2004, respectively). Krathom, a local addictive plant, and cannabis were the most commonly used illicit substances on a lifetime basis with prevalences of 2.3%, 2.8%, 4.9% (p<0.01) and 2.6%, 2.3%, 3.4% (p>0.05) in the surveyed years. The rates of alcohol consumption in the past 30 days were 19.3%, 17.3% and 15.2% (p>0.05) while smoking rates were 14.6%, 8.8% and 10.8% (p<0.05). The significant correlates of current illicit substance use were surveyed year, male, vocational school, school level, and school performance. CONCLUSION:The problem of substance abuse is increasing among Thai adolescents. School-based interventions seem desirable, especially in boys, vocational and public school students.