The efficacy of furazolidone for treatment of intestinal microsporidiosis due to Enterocytozoon bieneusi was studied in three patients with AIDS. All patients had chronic diarrhoea and weight loss. Mean CD4 cell count was 34.6/mm3. A course of furazolidone (100 mg orally four times a day) was given for 20 days. The drug was well tolerated and neither side effects nor alterations in the laboratory parameters were noted. Diarrhoea ceased within a mean of 12 days of starting treatment and clearance of microsporidian shedding in stool was observed. In one of the patients, however, symptomatic microsporidiosis recurred. Therefore furazolidone seems to have a transient but significant effect on intestinal infection due to Enterocytozoon bieneusi.