BACKGROUND: In December 2012, Australia introduced world-first legislation mandating plain packaging for all tobacco products. To date, there is very little evidence on youth responses to the changed packs. AIM: To assess attitudes towards, and responses to, tobacco plain packs preimplementation and postimplementation. METHODS: The Tobacco Promotion Impact Study (TPIS) was a yearly cross-sectional telephone survey of adolescents and young adults (12-24 years) from the states of New South Wales (NSW) and Queensland, conducted at three time points preimplementation (June 2010; June 2011; June 2012) and one time point postimplementation (June 2013; total n=8820). RESULTS: There were significant increases in support for plain packaging from preimplementation to postimplementation for: never smokers (56% in 2012 vs 63% in 2013; OR=0.77, 95% CI 0.65 to 0.90, p=0.001), experimenters/ex-smokers (55% in 2012 vs 72% in 2013; OR=0.51, 95% CI 0.38 to 0.68, p<0.001) and current smokers (35% in 2012 vs 55% in 2013; OR=0.49, 95% CI 0.32 to 0.75, p=0.001). At postimplementation, 16% of never smokers reported that plain packaging made them less likely to try smoking and 18% of experimenters/ex-smokers reported that plain packaging made them less likely to smoke again. Youth were significantly less likely to have anticipated these responses preimplementation (never smokers: 8% in 2011; OR=0.43, 95% CI 0.28 to 0.65, p<0.00; experimenters/ex-smokers: 11%; OR=0.65, 95% CI 0.52 to 0.82, p<0.001). At postimplementation, 34% of smokers reported a quitting-related response to plain packaging (tried to quit or thought about quitting); the proportion who anticipated such a response preimplementation was significantly less (14% in 2011; OR=0.33, 95% CI 0.20 to 0.53, p<0.001). 28% of smokers reported a social denormalisation response at postimplementation (hid their pack from view, used a case to cover their pack, felt embarrassed); the proportion who anticipated such a response preimplementation was significantly less (9% in 2011; OR=0.24, 95% CI 0.14 to 0.42, p<0.001). CONCLUSIONS: The actual response of youth to plain packaging was greater than anticipated prior to their introduction, and support for plain packaging increased from preimplementation to postimplementation among all groups of youth. Jurisdictions planning to implement plain tobacco packaging should be encouraged by these findings.