Purpose Trends in school-based differences of smoking prevalence among high school students in Japan was analysed to assess social inequalities in health among adolescents.

Methods Cross-sectional nationwide surveys were conducted periodically. High schools were randomly sampled from throughout Japan in 1996, 2000, 2004, and 2008. All enrolled students in sampled schools were asked smoking and drinking behaviour. The number of schools participated in the survey was 80 junior high schools and 73 senior high schools in 1996 survey. The values were 99 and 77 in 2000, 92 and 87 in 2004, and 92 and 80 in 2008 survey, respectively. For assessing the differences in prevalence, the coefficient of variation (CV) was used.

Results Smoking prevalence among students has decreased for both sexes. According to the lowering smoking prevalence, the variance of between-school differences in smoking prevalence has shrunk, however CV of the experiment rate, current smoking (smoked at least 1 day of the 30 days preceding the survey), and daily smoking has increased for both sexes and both junior and senior high school. For example, CV of current smoking in junior high school boys was 0.51 in 1996, 0.52 in 2000, 0.66 in 2004, and 0.92 in 2008. The CV in junior high school girls was 0.62 in 1996, 0.63 in 2000, 0.93 in 2004, and 0.95 in 2008.

Conclusions Although the average (mean or median) of smoking prevalence by school decreased, the between-school differences have increased. This suggests that expansion of social inequalities in adolescent health may be occurring