Exposure to tobacco smoke has been shown to have a profound effect on DNA methylation patterns in many recent studies. Site-specific DNA methylation in response to smoking is dynamic and may change over time. Although studies have investigated the extent to which changes in DNA methylation that are associated with smoking, persist after smoking cessation (1-3), the length of time required for smoking to have an impact on DNA methylation is currently unstudied. A number of studies, focused on young people and adolescents, have shown that site-specific DNA methylation is associated with smoking (4-9), suggesting that methylation changes may be induced in individuals with a relatively short smoking history. However, the previous studies have typically evaluated methylation at a few CpG sites and have not modelled methylation changes with time since smoking initiation. This study aims to assess the dynamics of smoking-induced genome-wide methylation changes with time since smoking initiation in a cohort of adolescents from the Avon Longitudinal Study of Parents and Children (ALSPAC).