Enterocytozoon bieneusi is a protozoa belonging to the Microsporidia family which prevalence has increased in AIDS patients. Although diagnosis is performed by the demonstration of the parasite in the epithelium of the small intestine by light and electron microscopy, techniques allowing diagnosis from stools or duodenal or biliary aspirates have recently been described. Three cases of intestinal microsporidiosis diagnosed by the mentioned method are reported. The patients were 3 males with chronic diarrhea of several months of evolution with an important ponderal loss. All were in advanced stages of HIV infection with CD4-lymphocyte counts lower than 0.1 x 10(9)/l. In all the patients in whom intestinal absorption tests were performed these were found to be altered. One of the patients presented concommitant cholestasis with parasitation by E. bieneusi being demonstrated as by the biliary route in this patient. Confirmation of infection by E. bieneusi was performed in the 3 cases by electron microscopy study of stools. A review of intestinal microsporidiosis in AIDS patients is carried out and the therapeutic possibilities available for this infection are discussed.