Cystic Fibrosis (CF) is a genetic disease leading to sticky mucus. We used MRI to characterise the effect of CF on gastrointestinal function, comparing people with CF to matched healthy controls. People with CF had slower orocaecal transit times. No change in gastric emptying rate was apparent but more free water was present in their small bowel with reduced small bowel motility and a reduced gastro-ileal reflex. Some images suggested increased bacterial load in the small bowel. CF colons were larger. These findings are consistent with sticky chyme impeding ileal emptying into the colon, causing obstruction to flow, and constipation.