Crohn's disease (CD) is an inflammatory disorder of the gastrointestinal tract. The cause is unknown but clinical studies indicate that luminal factors and bacteria in the gut are involved in disease aetiology, although the infective organisms that have been implicated in CD, such as atypical mycobacteria and helicobacters, do not explain the various lesions that occur throughout the digestive tract. However, there is some evidence to suggest that mycoplasmas may be associated with CD. For example, mycoplasmas have been linked to colitis in dogs, while intracellular structures similar in morphology to these organisms have been observed in epithelial cells by electron microscopy of CD tissue. Lipids such as cholesterol are required for the growth of some mycoplasmas and dietary studies where the fat content in enteral formula diets was reduced have been demonstrated to have significant therapeutic value in patients with CD. Further support for mycoplasmal involvement in CD derives from improvements in inflammatory indices following the use of antibiotics known to be effective against mycoplasma infections, such as clarithromycin, rifabutin and clofazimine.