Objectives To investigate the links between health warning labels (WLs) on cigarette packets and relapse among recently quit smokers. Design Prospective longitudinal cohort survey. Setting Australia, Canada, the UK and the USA. Participants 1936 recent ex-smokers (44.4% male) from one of the first six waves (2002-2007) of the International Tobacco Control 4-Country policy evaluation survey, who were followed up in the next wave. Main outcome measures Whether participants had relapsed at follow-up (approximately 1 year later). Results In multivariate analysis, very frequent noticing of WLs among ex-smokers was associated with greater relapse 1 year later (OR: 1.52, 95% CI 1.11 to 2.09, p<0.01), but this effect disappeared after controlling for urges to smoke and self-efficacy (OR: 1.29, 95% CI 0.92 to 1.80, p¼0.135). In contrast, reporting that WLs make staying quit ‘a lot’ more likely (compared with ‘not at all’ likely) was associated with a lower likelihood of relapse 1 year later (OR: 0.65, 95% CI 0.49 to 0.86, p<0.01) and this effect remained robust across all models tested, increasing in some. Conclusions This study provides the first longitudinal evidence that health warnings can help ex-smokers stay quit. Once the authors control for greater exposure to cigarettes, which is understandably predictive of relapse, WL effects are positive. However, it may be that ex-smokers need to actively use the health consequences that WLs highlight to remind them of their reasons for quitting, rather than it being something that happens automatically. Ex-smokers should be encouraged to use pack warnings to counter urges to resume smoking. Novel warnings may be more likely to facilitate this. [ABSTRACT FROM AUTHOR]