Impaired intestinal mucosal barrier function may be involved in the pathogenesis of diarrhoea-predominant IBS (D-IBS) accompanied by persistent low-grade intestinal inflammation. Attenuating the inflammation and preserving mucosal barrier function may be a potential therapeutic target in D-IBS. This study investigates whether probiotic fermented milk containing multistrain lactic acid bacteria (LAB) can improve intestinal mucosal barrier function in D-IBS. 30 Chinese adults with D-IBS and 12 asymptomatic controls were randomized (1:1 ratio) to consume a probiotic fermented milk (containing Streptococcus thermophilus, Lactobacillus bulgaricus, Lactobacillus acidophilus and Bifidobacterium longum) or a placebo drink twice daily half an hour before meals for 4 weeks. IBS symptoms and intestinal permeability were evaluated at weeks 0 and 4. Before treatment, intestinal permeability of the 30 patients with D-IBS was increased compared to normal controls. After treatment, intestinal permeability in the probiotics group decreased significantly, compared to the placebo group. IBS symptoms score in the probiotics group improved compared with baseline values, but not in the placebo group. This study found that small intestinal permeability was significantly increased in D-IBS patients compared with normal controls. The results indicate that multistrain LAB was associated with the improvement in intestinal barrier function as measured by a reduction in small bowel permeability, in turn accompanied by relief of IBS symptoms, which suggests that increased intestinal permeability may partially contribute to the pathogenesis of IBS symptoms. The mechanisms of increased intestinal permeability involvement in D-IBS were lacking.