Blastocystis hominis (B. hominis) is a parasite of uncertain role in human disease. It may be identified during a workup for gastrointestinal symptoms, usually in stools. The clinical consequences of B. hominis infection are mainly diarrhea and abdominal pain as well as nonspecific gastrointestinal symptoms such as nausea, anorexia, vomiting, weight loss, lassitude, dizziness, and flatulence. Case reports and series have suggested a pathogenic role of B. hominis in causing intestinal inflammation. Also some studies have suggested that inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS) are associated with B. hominis infection. The investigators indicate that the stools of all patients presenting with IBD or IBS should be examined, and culture methods for B. hominis carried out. Invasion and mucosal inflammation of the intestine with B. hominis have been observed in studies of gnotobiotic guinea pigs. The transmission, pathogenicity, culture characteristics, taxonomy, life cycle, biochemistry and molecular biology of B. hominis remain unclear. More studies are necessary for this parasite.