BACKGROUND/AIMS: Irritable bowel syndrome (IBS) is a functional gastrointestinal (GI) tract disorder that has heterogeneous clinical presentations such as abdominal pain, diarrhea, constipation, and abdominal distension. It is known that several mechanisms are involved in the pathogenesis of IBS. Probiotics may target one or more pathophysiologic pathways in IBS and may improve the symptoms of IBS. However, the results of studies about probiotics on IBS are controversial. Therefore, the aim of this study was to evaluate the effect of probiotics on GI symptoms and intestinal gas volume changes in patients with IBS. METHODS: Forty patients were randomly allocated to be treated with medilac DS(R) (Bacillus subtilis, Streptococcus faecium) (n=20) or placebo (n=20) in a double-blind, prospective manner. The change in intestinal gas volume and symptom scores after 4-week treatment were evaluated for the efficacy. RESULTS: There was no significant difference in bloating, frequency of gas expulsion, frequency of defecation, and hardness of stool before and after the treatment. However, the severity of abdominal pain and the frequency of abdominal pain decreased significantly in medilac DS(R)group (2.4+/-1.3 cm/day -> 1.6+/-1.6 cm/day, 1.7+/-1.3/day -> 1.0+/-1.0/day) (p=0.044, p=0.038), but not in placebo group (2.1+/-2.0 cm/day -> 1.8+/-2.1 cm/day, 1.3+/-1.2/day -> 1.4+/-1.9/day). In both groups, intestinal gas volume at baseline, after 2-week treatment, and after 4-week treatment did not show significant change. Medilac DS(R)was well tolerated without adverse events. CONCLUSIONS: Medilac DS(R)is a safe and useful probiotic agent for the treatment of abdominal pain in patients with IBS.