- BACKGROUND: Post-infectious irritable bowel syndrome is a common clinical phenomenon of uncertain aetiology. AIM: To test the association between intestinal permeability and irritable bowel syndrome symptoms 2 years after a large waterborne outbreak of bacterial gastroenteritis. METHODS: Consecutive adults with Rome I irritable bowel syndrome and controls without irritable bowel syndrome attending a community clinic were enrolled. Intestinal permeability was measured as the ratio of fractional urinary excretions of lactulose and mannitol, and compared among cases vs. controls and predictors of abnormal intestinal permeability were assessed. RESULTS: A total of 218 subjects (132 irritable bowel syndrome cases and 86 non-irritable bowel syndrome controls) completed the study protocol. About 27 (12%) had been diagnosed with the irritable bowel syndrome before the outbreak and 115 (53%) had been ill during the outbreak. Lactulose-mannitol ratios were increased among cases vs. controls (Mann-Whitney mean rank 118.8 vs. 95.3, P = 0.007), and cases were more likely to have a ratio >0.020 (P = 0.007). Among cases, those with increased intestinal permeability were more likely to report increased stool frequency. Both irritable bowel syndrome symptoms and male gender, but not diarrhoeal illness during the outbreak, were significant predictors of abnormal permeability. CONCLUSIONS: Irritable bowel syndrome symptoms are associated with a subtle increase in intestinal permeability irrespective of prior gastroenteritis. This may improve understanding of the aetiology of both sporadic and post-infectious irritable bowel syndrome.