A double-blind randomized trial in China has assessed the effect of operational bias in the use of computer-aided detection (CAD) systems for polyp detection. A total of 962 patients presenting for diagnostic and screening colonoscopy were randomly allocated to colonoscopy with a CAD or a sham system; both patient and operator were unaware of the assigned group. The adenoma detection rate was significantly greater in the CAD group than the sham group (P = 0.03). Polyps detected by the CAD system but not the endoscopist had difficult-to-recognise characteristics (small, flat, isochromatic and unclear boundaries).