Background: Undergoing a colonoscopy can cause anxiety in patients and this is something which has not been closely studied.Objective: To determine the frequency and intensity of anxiety prior to a colonoscopy and the factors which are related to the procedure.Methods: This is a prospective study of patients undergoing outpatient colonoscopy in our hospital. Anxiety was assessed using a visual analogue scale of 0 to 100. The severity of anxiety was rated as mild (1-29), moderate (30-79) or severe (80-100).Results: Three hundred and twenty-seven patients completed the study, of whom 154 (47.1%) were men with a median age of 54 years (p25-75: 45-65). Three hundred and nine (94.5%) patients were found to suffer a certain degree of anxiety. The median value on the visual analogue scale was 31 (p25-75: 10-53). Anxiety levels were mild in 136 patients (44%), moderate in 141 (45.6%) and severe in 32 (10.4%). Greater anxiety was associated with female patients (mean 40.38 vs 31.99, p = 0.01) and a poorly tolerated previous colonoscopy (mean 50.67 vs 28.44, p = 0.01) and correlated inversely with age (r = -0.170, p = 0.02).Conclusions: Colonoscopy causes some degree of anxiety in most patients. Being female, younger and having experienced poor tolerance to a previous scan are associated with greater degrees of anxiety. These findings should be taken into account in the implementation of measures to improve the quality and tolerance of colonoscopy.