Background The NHS Bowel Cancer Screening Programme uses biennial guaiac faecal occult blood testing (FOBt) to screen men and women initially aged 60–69 for colorectal cancer. The programme provides a valuable opportunity for screening-related epidemiological studies.

Aim Assess the impact of a research questionnaire on uptake of FOBt screening.

Study 1 10 940 participants (5470 in each arm) invited for screening by the Midlands & North West Bowel Cancer Screening Hub were randomised to receive or not receive a study questionnaire pack (questionnaire, patient information sheet, consent forms and reply-paid envelope) with their screening test kit. Screening uptake was ascertained from screening programme records and a χ² test used to assess any association between receiving a questionnaire and screening uptake. Screening uptake was significantly lower in those sent a questionnaire than those who were not (50.8% vs 55.2%, p<0.001).

Study 2 36 225 participants were batch-randomised to receive or not receive a questionnaire pack 2–3 days after their FOBt screening kit mailing by the Midlands & North West Screening Hub (6168 receiving and 13 158 not receiving questionnaires) or Southern Screening Hub (5801 receiving and 11 098 not receiving questionnaires). Screening uptake did not differ between those receiving or not receiving questionnaire packs [Midlands & North West: 56.7% vs 56.6% (p=0.9); Southern: 53.4% vs 53.4% (p=1)].

Conclusion Including research questionnaires within FOBt mailings resulted in a significant screening uptake reduction. However, sending the same questionnaires 2–3 days after FOBt kits did not. These findings may have implications for future research within the screening programme.