Intestinal microsporidiosis is caused by the protozoan parasites Enterocytozoon bieneusi and Encephalitozoon intestinalis. The disease has been described within the past decade and is found predominantly in acquired immunodeficiency syndrome (AIDS) patients in association with diarrhea. There have been rare reports of infections in immunocompetent patients. Both species of microsporidia invade and multiply within the enterocytes of the small intestine; Encephalitozoon intestinalis also causes a disseminated infection. Electron microscopy has been the mainstay of diagnosis, but improved noninvasive methods of detecting spores in feces are in development. The biology and pathogenicity of the parasites and the pathology and treatment of the disease are poorly understood.