Irritable bowel syndrome (IBS) is characterized by abdominal pain and alterations in bowel habits. Several pathogenetic factors, such as altered intestinal motility, visceral hypersensitivity, serotonin system abnormalities and psychic disturbances have been identified. Recently, a pathogenetic role of intestinal microflora has been shown in IBS: viral or bacterial infection can trigger post-infectious IBS; some patients have small intestinal bacterial overgrowth; the composition of patients' enteric flora is altered; and minimal inflammatory changes, consistent with the pro-inflammatory role of bacteria, have been demonstrated. Probiotics may, therefore, offer a rational therapeutic approach to IBS. The data available on the use of probiotics in IBS are still limited and results of controlled clinical trials are contradictory because they have been performed using different species, dosages, treatment durations and end-points for results evaluation. A critical evaluation of the therapeutic role of the various probiotics in IBS is presented in this article.