A survey on microsporidiosis in individuals with AIDS presenting chronic diarrhoea was carried out in Italy, over a four-year period (1992-1995). Three out of 72 (4.2%) individuals were found positive, on intestinal biopsies, for Enterocytozoon bieneusi by light microscopy and transmission electron microscopy (TEM). Sixteen individuals with AIDS, from a second group of subjects, were confirmed positive, by TEM, for intestinal microsporidiosis due to Enterocytozoon bieneusi. Of these 19 cases, 10 (52.6%) were homosexual men. Two of these individuals, under albendazole treatment, showed also spores with unusual features. The prevalence of intestinal microsporidiosis (12-50%) reported in European countries, Australia and North America, where homosexuality is the major HIV risk factor (63-77%), is higher than in Italy, where homosexual men represent only 16% of the total number of AIDS cases.