Irritable bowel syndrome (IBS) is a very common chronic gastrointestinal disorder. Existing therapies for IBS are still far from satisfactory and new therapies are being constantly sought. Probiotics are promising candidate for the prevention and treatment of IBS. However, its efficacy and mechanism is still unclear. The current study aims to evaluate the effects of a combination probiotic, VSL#3, on symptoms and rectal sensitivity in patients with IBS. The underlying mechanism will also be investigated. We will recruit forty patients with Rome III IBS and randomized them, in a parallel group, double-blinded design, to take four VSL#3 capsules or placebos, twice daily, for six weeks. Before and after treatment, patients will be assessed for their symptoms, rectal sensitivity, as well as saliva and fecal melatonin levels. Bowel symptom diary and weekly satisfactory relief of bowel symptoms will also be recorded and evaluated.