Saccharomyces boulardii is a non-pathogenic yeast which has been used as both a preventive and therapeutic agent for the treatment of a variety of diarrhoeal diseases. The studies with animal models and evidence from human volunteers and patients indicate a profile which is effective in the therapy of diarrhoea and is remarkably safe for oral ingestion. The pharmacokinetic data demonstrate that 5. boulard reaches a steady-state concentration quickly and maintains a high stable level as long as the yeast is taken daily. Once the agent is discontinued, S. boulard is quickly eliminated from the colon. Clinical trials studying antibiotic-associated diarrhoea, nasogastric-tube alimentation diarrhoea, Clostridium difficile-disease, acute diarrhoea and chronic diarrhoea in HI V-infected patients are reviewed.