In 1990, a nationwide random survey of smoking habits among junior and senior high school students was conducted in order to study the smoking prevalence among adolescents in Japan. The self-administered anonymous questionnaires on personal smoking status were sent to all students in sample schools. Seventy junior high schools and thirty-three senior high schools responded with 57,566 questionnaires returned, of which 57,189 were subjected to analysis after excluding incomplete and inconsistent questionnaires. The results were as follows; 1) The percentage of experimenters who had smoked even once was 23.3% among boys and 8.7% among girls in the first grade of junior high school (seventh grade). The percentage increased with school grade and reached nearly half of boys and nearly one sixth of the girls in the third grade of senior high school (twelfth grade). 2) The smoking rate was much higher among boys than girls, and the current smoking rate in the first grade of junior high school was 4.0% for boys and 1.5% for girls. Current smoking rate increased with grade, reaching 25.5% for boys and 4.9% for girls in the third grade of senior high school. 3) While the percentage of regular smokers in the first grade of junior high school was less than 1% for both sexes, it went up to 20.3% for boys and 2.6% for girls in the third grade of senior high school. 4) In current smokers, the proportion of boys smoking 1-4 cigarettes per day decreased and that of those smoking 15-19 cigarettes, and 20 or more cigarettes increased with increasing school grade.(ABSTRACT TRUNCATED AT 250 WORDS)