Probiotics are living organisms which, when ingested, have a beneficial therapeutic effect. Examples are bacteria, especially Lactobacillus rhamnosus GG, and the yeast Saccharomyces boulardii. Controlled trials indicate a benefit of both of these in the prevention of antibiotic-associated diarrhoea. Other less effective probiotics are Lactinex, Enterococcus faecium and bifidobacteria. In the difficult clinical problem of recurrent Clostridium difficile disease, S. boulardii as an adjunct to antibiotics has shown benefit in controlled trials. There is, however, less convincing evidence for the efficacy of Lactobacillus GG in this disease. Additional controlled trials and safety studies are needed before there can be a widespread endorsement of probiotics for these two conditions.