Background: Loperamide improves anorectal function in patients with chronic diarrhoea. We wished to investigate whether the prodrug loperamide oxide has similar effects. Methods: Eleven patients with chronic diarrhoea and faecal incontinence participated in a randomized, placebo-controlled, double-blind, crossover study of the effects of loperamide oxide (4 mg twice daily for 1 week). Results: Loperamide oxide reduced wet stool weight and improved the patients' ratings of symptoms. Mouth-to-caecum transit time was not altered, but whole-gut transit time was prolonged. There were limited effects on anorectal function, but the mean minimum basal pressure mainly contributed by the internal anal sphincter (IAS) was increased, as was the mean volume infused before leakage occurred in the saline continence test. Conclusion: Loperamide oxide is effective in the treatment of diarrhoea with faecal incontinence; normalization of colon transit time and an increase in the tone of the IAS seem to be the main determinants of efficacy.