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Background: Vitamin D deficiency has been associated or implicated with the pathophysiology of the gastrointestinal conditions inflammatory bowel disease and colorectal cancer, as well as with depression. No trials or epidemiology studies to date have investigated a link with irritable bowel syndrome (IBS). A single case report has suggested a benefit in IBS of vitamin D supplementation. We hypothesised that IBS participants with vitamin D insufficiency would benefit from repletion in terms of their IBS symptoms. We undertook a pilot trial to provide data to support a power calculation and to justify a full trial., Methods: This was a randomised, double blinded, three-arm parallel design trial of vitamin D, placebo or a combination of vitamin D and probiotics. Participants were further stratified according to whether they were vitamin D replete or insufficient. Vitamin D status was determined by blood test at baseline and exit; IBS symptoms were assessed by validated questionnaire; dietary intakes were assessed by food frequency questionnaire., Results: A significant proportion of the IBS population were vitamin D deficient, such that the replete stratum could not be adequately recruited. There was a significant association in the baseline data between circulating vitamin D level and quality of life ("How much has IBS affected your life?"). Supplementation significantly improved vitamin D level versus placebo. IBS symptoms were not significantly improved in this pilot, although a power calculation was enabled from the intervention data., Conclusions: The IBS population exhibits significant levels of vitamin D insufficiency and would benefit from screening and possible supplementation. The impact of IBS on quality of life may be reduced by vitamin D level. Future trials should have a sample size of over 97., Trial Registration Number: ICTRN 6116003917.