Objective Conventional colonoscopy (CC) is considered the reference standard for detection of colorectal neoplasia, but it can still miss a substantial number of adenomas. The use of a transparent plastic cap may improve colonic visualisation. Cap-assisted colonoscopy (CAC) was compared with CC for adenoma detection. Secondary outcomes were caecal intubation time, caecal intubation rate and the degree of discomfort of colonoscopy. Design This is a parallel, randomised, controlled trial at two centres. Asymptomatic participants (aged 50-75 years) in a primary colonoscopy screening programme were consecutively invited. Consenting subjects were 1: 1 randomised to either CAC or CC. All colonoscopies were performed by experienced endoscopists (>= 1000 colonoscopies) who were trained in CAC. Colonoscopy quality indicators were prospectively recorded. Results A total of 1380 participants were randomly allocated to CC (N = 694) or CAC (N = 686). Caecal intubation rate was comparable in the two groups (98% vs 99%; p = 0.29). Caecal intubation time was significantly lower in the CAC group: 7.7 +/- 5.0 min with CAC vs 8.9 +/- 6.2 min with CC (p<0.001) (values mean +/- SD). Adenoma detection rates of all endoscopists were >= 20%. The proportion of subjects with at least one adenoma was similar in the two groups (28% vs 28%; RR 0.98; 95% CI 0.82 to Conclusion CAC does not improve adenoma detection, but does reduce caecal intubation time by more than 1 min and does lessen the degree of discomfort during colonoscopy.