Disseminated microsporidiosis due to the newly described species Septata intestinalis in nine patients infected with human immunodeficiency virus is described. All patients were male homosexuals; the mean age was 41 years (range, 35-58 years). They were all severely immunocompromised, with a mean CD4 lymphocyte count of 15/mm3 (range, 0-32/mm3). Infection by S. intestinalis was seen in duodenal biopsy specimens from all patients, and dissemination was demonstrated by the presence of microsporidial spores in urine (9 of 9 patients), sinonasal secretions and/or nasal mucosal biopsy specimens (6 of 6), and sputum (6 of 6). Seven patients were treated with albendazole (400 mg twice daily), resulting in significant dissipation or complete resolution of diarrhea for six patients and abatement of symptoms for the six patients with chronic rhinosinusitis. There was a parallel parasitological response, with clearance of S. intestinalis infection from almost all sites.