Background and study aims: Removal of colonic polyps prevents progression of colonic neoplasia. Miss rates of polyps range from 5% to 32%. The effect of colonic contractility on polyp detection has not been studied adequately. Hyoscine butylbromide results in colonic spasmolysis and may improve polyp detection. Patients and methods: Patients undergoing colonoscopy for standard indications were included and randomized to receive either 20mg hyoscine butylbromide or placebo at cecal intubation. Operators were blind to the intervention. Data on indication, preparation, sedation, colonoscope type, times of insertion/withdrawal, polyps, and failure were recorded. The primary end point was the number of polyps detected per patient. Secondary endpoints were adenoma detection rate and polyp detection rate. Results: A total of 303 patients received hyoscine butylbromide and 298 received placebo. More polyps per patient were identified in the hyoscine group than in the placebo group (0.91 vs. 0.70; P=0.044). Adenoma detection rate and polyp detection rate were higher in the hyoscine arm but not significantly different (27.1% vs. 21.8% [P=0.13] and 43.6% vs. 36.6% [P=0.08], respectively). After adjusting for confounding variables, the odds of detecting any polyp were 1.56 higher in the hyoscine than the placebo group (95% confidence interval [CI] 1.09-2.21, P=0.014). The adjusted odds of detecting any adenoma were 1.62 higher in the hyoscine group compared with the placebo group (95%CI 1.09-2.42, P=0.017). There were no differences in baseline characteristics between the groups. No adverse colonoscopy-related events were recorded. One patient experienced transient tachycardia without sequelae. Conclusions: Hyoscine butylbromide administered at the cecum aids polyp detection. Further studies are required to determine the contribution of colonic spasm to polyp miss rates.