Colonoscopy is a procedure often experienced as uncomfortable and worrying. Music has been reported to reduce discomfort during colonoscopy; however, no study in a Swedish setting has been found. The purpose of this randomized controlled trial was to analyze the effects of sedative music on patients' experience of anxiety, pain, relaxation, and well-being during colonoscopy. Prior to colonoscopy, adult patients (n = 120), aged 18–80 years, were randomly assigned to either an intervention group (n = 60) who listened to sedative instrumental music with 60–80 beats per minute during the colonoscopy or a control group. After the colonoscopy, both groups completed a questionnaire on anxiety, the State Trait Anxiety Inventory, and an anxiety Visual Analogue Scale. Pain, relaxation, and well-being were also measured with Visual Analogue Scales. Women in the intervention group had a lower level of anxiety during the colonoscopy than those in the control group (p = .007) and well-being was significantly higher in the intervention group, especially among men, than in the controls (p = .006 and p = .025, respectively). Men in the intervention group were more relaxed during the colonoscopy than those in the control group (p = .065). Listening to sedative music decreased anxiety among women and increased well-being among men during colonoscopy.