To review the rationale and current evidence for the efficacy of probiotics in the treatment of irritable bowel syndrome (IBS), the following specific questions are addressed from a review of the published literature: Are bacterial flora altered in IBS? Is infection a significant risk factor for IBS? How prominent is the inflammation in IBS? Do probiotics change bacterial flora? Do probiotics adhere to mucosa? What are the potential mechanisms of action of probiotics in IBS: immune function, motor, secretory, sensory, and fermentation? What is the efficacy of probiotics in IBS?