Mild or severe episodes of antibiotic-associated diarrhea (AAD) are common side effects of antibiotic therapy. The incidence of AAD differs with the antibiotic and varies from 5 to 25%. The major form of intestinal disorders is the pseudomembranous colitis associated with Clostridium difficile which occurs in 10-20% of all AAD. In most cases of AAD discontinuation or replacement of the inciting antibiotic by another drug with lower AAD risk can be effective. For more severe cases involving C. difficile, the treatment of diarrhea requires an antibiotic treatment, with glycopeptides (vancomycin) or metronidazole. Another approach to AAD treatment or prevention is based on the use of non-pathogenic living organisms, capable of re-establishing the equilibrium of the intestinal ecosystem. Several organisms have been used in treatment or prophylaxis of AAD such as selected strains of Lactobacillus acidophilus, L. bulgaricus, Bifidobacterium longum, and Enterococcus faecium. Another biotherapeutic agent, a non-pathogenic yeast, Saccharomyces boulardii has been used. In animal models of C. difficile colitis initiated by clindamycin, animals treated with S. boulardii (at end of vancomycin therapy) had a significant decrease in C. difficile colony-forming units, and of toxin B production. In several clinical randomised trials (versus placebo), S. boulardii has demonstrated its effectiveness by decreasing significantly the occurrence of C. difficile colitis and preventing the pathogenic effects of toxins A and B of C. difficile. It has been shown to be a safe and effective therapy in relapses of C. difficile colitis. A good response has been seen in children with AAD, treated by S. boulardii only. In ICUs prevention of AAD remains based on limitation of antibiotic overuse and spread of C. difficile or other agents of AAD should be prevented by improved hygiene measures (single rooms, private bathrooms for patients, use of gloves and hand washing for personnel). In addition the increasing use of biotherapeutic agents such as S. boulardii should permit the prevention of the major side effect of antibiotics, i.e. AAD in at risk patients.