Background and study aims: Uptake of flexible sigmoidoscopy screening in the English Bowel Scope Screening (BSS) Programme is low. The aim of this study was to test the impact of a nonparticipant reminder and theory-based leaflet to promote uptake among former nonresponders (previously did not confirm their appointment) and nonattenders (previously confirmed their appointment but did not attend). Patients and methods: Eligible adults were men and women in London who had not attended a BSS appointment within 12 months of their invitation. Individuals were randomized (1:1:1) to receive no reminder (control), a 12-month reminder plus standard information booklet (TMR-SIB), or a 12-month reminder plus bespoke theory-based leaflet (TMR-TBL) designed to address barriers to screening. The primary outcome of the study was the proportion of individuals screened within each group 12 weeks after the delivery of the reminder. Results: A total of 1383 men and women were randomized and analyzed as allocated (n = 461 per trial arm). Uptake was 0.2 % (n = 1), 10.4 % (n = 48), and 15.2 % (n = 70) in the control, TMR-SIB, and TMR-TBL groups, respectively. Individuals in the TMR-SIB and TMR-TBL groups were significantly more likely to attend screening than individuals in the control group (adjusted odds ratio [OR] 53.7, 95 % confidence interval [CI] 7.4 - 391.4, P