Blastocystis hominis is a protozoan organism frequently found in the human intestinal tract. Eleven consecutive patients with symptoms of enteritis and having B. hominis as the sole enteropathogen were studied in an attempt to define the association of blastocystosis with clinical disease. B. hominis could not be implicated as the etiologic agent of enteritis in any of these patients. All eleven had alternative (and usually noninfectious) explanations for their intestinal symptoms. There was no correlation between resolution of symptoms and either antiprotozoal therapy or disappearance of B. hominis from the stools. All prior reports associating B. hominis with human disease have been reviewed and provide no convincing proof of a causal relation. B. hominis is rarely, if ever, a human pathogen, and treatment directed at the eradication of B. hominis is not indicated.