Blastocystis hominis is a common inhabitant of the human bowel. It is now increasingly recognized as a potential cause of diarrhea. This article presents 12 cases of prolonged or recurrent diarrhea associated with B. hominis found in a large number. No other intestinal parasites were recognized. All patients responded to metronidazole. This report confirms that B. hominis may be a cause of intestinal disease.