Aim: To examine trends in socioeconomic differences in daily smoking among 15-year-old Danes between 1991-2014, using occupational social class as indicator of socioeconomic status. Methods: The study included 15-year-olds participating in seven Danish Health Behaviour in School-aged Children studies between 1991-2014, n = 8,641. The analyses focused on absolute socioeconomic differences (prevalence difference between low and high occupational social class) and relative socioeconomic differences communicated by odds ratio for daily smoking. Results: The prevalence of daily smoking declined from 18.6% in 1991 to 4.5% in 2014. Across all surveys, the prevalence was 8.9% in high, 12.8% in middle and 16.5% in low occupational social classes ( p < 0.0001). The absolute socioeconomic differences increased from 1991 to 2006 and declined thereafter. Across all survey years, the odds ratio (95% confidence interval) for daily smoking was 1.40 (1.19-1.65) in middle and 1.90 (1.56-2.32) in low versus high occupational social classes. The statistical interaction between occupational social class and survey year was significant ( p = 0.0404), suggesting increasing relative socioeconomic differences from 1991 to 2014. Conclusions : There was a substantial decline in daily smoking among 15-year-olds between 1991-2014 in all occupational social class groups. The prevalence of daily smoking was highest in the low occupational social class during the entire period. The absolute socioeconomic differences in daily smoking increased between 1991-2006 and declined thereafter. The relative socioeconomic differences increased over 1991-2014. Studies of change in socioeconomic differences over time should address both absolute and relative socioeconomic differences as they may result in different conclusions and because important improvement in prevalence patterns may be disguised by exclusive focus on changes in relative socioeconomic differences.