AIM:To determine the dose-related effects of a novel probiotic combination,I.31,on irritable bowel syndrome(IBS)-related quality of life(IBS-QoL).METHODS:A multicenter,randomized,double-blind,placebo-controlled intervention clinical trial with three parallel arms was designed.A total of 84 patients(53female,31 male;age range 20-70 years)with IBS and diarrhea according to Rome-Ⅲcriteria were randomly allocated to receive one capsule a day for 6 wk containing:(1)I.31 high dose(n=28);(2)I.31 low dose(n=27);and(3)placebo(n=29).At baseline,and 3and 6 wk of treatment,patients filled the IBSQoL,Visceral Sensitivity Index(VSI),and global symptom relief questionnaires.RESULTS:During treatment,IBS-QoL increased in all groups,but this increment was significantly larger in patients treated with I.31 than in those receiving placebo(P=0.008).After 6 wk of treatment,IBS-QoL increased by 18±3 and 22±4 points in the high and the low dose groups,respectively(P=0.041 and P=0.023vs placebo),but only 9±3 in the placebo group.Gutspecific anxiety,as measured with VSI,also showed a significantly greater improvement after 6 wk of treatment in patients treated with probiotics(by 10±2 and14±2 points,high and low dose respectively,P<0.05for both vs 7±1 score increment in placebo).Symptom relief showed no significant changes between groups.No adverse drug reactions were reported following the consumption of probiotic or placebo capsules.CONCLUSION:A new combination of three different probiotic bacteria was superior to placebo in improving IBS-related quality of life in patients with IBS and diarrhea.