The pathogenesis of irritable bowel syndrome (IBS) may have a a post-infectious inflammatory component, suggesting that altered gut bacterial flora are of relevance and that probiotics may be beneficial. This clinical trial examined the efficacy of an encapsulated probiotic in subjects with Rome II IBS. After a 2 week baseline, 362 female subjects were randomized to placebo or one of three doses of B. infantis 35624 once daily for 4 weeks. IBS symptoms were monitored daily and scored according to a 6-point Likert scale; stool frequency and form were also monitored daily. The primary efficacy variable was the abdominal pain score; secondary efficacy variables included other IBS symptom relief and quality of life.