BackgroundCancer Research UK ran a mass-media campaign for 6 weeks from Sept 1, 2022, to Sept 13, 2022, targeted to people aged 50 years and older and from a lower social-grade (C2DE) in Northern Ireland. The campaign was developed to target barriers to help-seeking for the target audience. With this evaluation, we aimed to compare reported barriers and self-reported help-seeking before and after the campaign to assess changes potentially related to the campaign. MethodsWe ran cross-sectional surveys that collected data from independent participants before the campaign (wave 1; n=1262; July 18–Aug 31, 2022) and after the campaign (wave 2; n=1250; Oct 3–Nov 3, 2022; N=2512). A recruitment agency recruited participants to be representative of Northern Ireland by age, gender, and social grade (Office for National Statistics, 2011; quota sampling). After wave 2, age range was 16–94 years (mean 51 years) and gender breakdown was 46% male (n=1152), 54% female (n=1356), and <1% other (n=3). Participants gave written informed consent, and the survey was administered using Computer Assisted Personal Interviewing (CAPI) at waves 1 and 2, and Computer Assisted Telephone Interviewing (CATI) at wave 1. Ethics approval was given by Queens University Belfast. Data were analysed in SPSS. Findings78% of the targeted audience (354/451) reported remembering the TV campaign. Of people who reported no symptoms, more reported “nothing would put them off or delay seeking medical attention” at wave 2 (447 [41%] of 1084 participants) than at wave 1 (221 [24%] of 936 participants), and this difference was significant (t(939.796)=2·243, p=0·025; 95% CI 0·083 to 1·244). Participants who reported experiencing a symptom (wave 1 n=186; wave 2 103) were less likely to report that they did not seek help from their GP or doctor in wave 2 (n=63 [61%]) than in wave 1 (n=157 [85%]); however, this difference was not significant (t(214.167)=1·133, p=0·258, 95% CI –0·051 to 0·188). InterpretationDecreases in barriers reported “nothing putting them off seeking help” and decreases in not seeking help from a GP or doctor to help seeking for actual symptoms experienced suggests this campaign might have resulted in some positive shifts in self-reported help-seeking behaviours for actual symptoms. However, these findings should be interpreted in light of the limitations of this research. Although this was a large sample, additional consideration should be given to the use of independent samples (rather than repeated surveys with the same population), self-reported data, and different data collection methods. FundingCommunity Foundation for Northern Ireland through the Department of Health Cancer Charities Fund.