OBJECTIVE The objective was to study the relationship between mental health and lifestyles of adolescents using samples representative of Japanese adolescents nationwide. METHODS The survey was conducted between December 2004 and January 2005 among students enrolled in randomly selected junior and senior high schools. Self-administered questionnaires addressed lifestyles, sleeping habits, and mental health status. Of 103 650 questionnaires collected, 85 158 were analyzed. RESULTS Population characteristics associated with poor mental health were being female, being a senior high school student, skipping breakfast, not participating in extracurricular activities, not consulting parents about personal matters, parental smoking, students' smoking or alcohol use, poor subjective sleep assessment, and short or long sleeping duration. CONCLUSIONS Smoking and anxiety disorders are associated with an elevation in acculturative stress in adolescents. Prepossessing adolescents are in subclinical depression. Results suggest that lifestyles approaches in preventions that target students can be effective in reaching high-risk populations.