Probiotics have been defined by The Food Agricultural Organization/World Health Organization (FAO/WHO) as "live microorganisms which when administered in adequate amounts confer a health benefit to the host." They have been used for centuries in the form of dairy-based fermented products, but the potential use of probiotics as a form of medical nutrition therapy has not received formal recognition. A detailed literature review (from 1950 through February 2004) of English-language articles was undertaken to find articles showing a relationship between probiotic use and medical conditions. Medical conditions that have been reportedly treated or have the potential to be treated with probiotics include diarrhea, gastroenteritis, irritable bowel syndrome, and inflammatory bowel disease (Crohn's disease and ulcerative colitis), cancer, depressed immune function, inadequate lactase digestion, infant allergies, failure-to-thrive, hyperlipidemia, hepatic diseases, Helicobacter pylori infections, genitourinary tract infections, and others. The use of probiotics should be further investigated for possible benefits and side-effects in patients affected by these medical conditions.