Blastocystis hominis is a common intestinal parasite of humans in the tropics whose pathogenic role is in dispute. Its presence has been reported in a variety of intestinal disorders resembling irritable bowel syndrome (IBS) such as diarrhea, anorexia, and flatulence. We have therefore investigated a possible link between IBS and blastocystosis by determining IgG antibody levels to B. hominis in patients with IBS. Levels of IgG antibodies were significantly elevated in patients with IBS compared with asymptomatic controls (P < 0.0001, by Student's t-test) in both B. hominis stool culture-positive and stool culture-negative IBS patients. When IgG antibodies were divided into their respective subclasses, only IgG2 levels were significantly increased in IBS patients compared with asymptomatic controls, indicating that the predominant response in these patients may be directed to carbohydrate antigens. The diagnostic usefulness of this test in IBS patients remains to be established because these data are only suggestive of a possible link between B. hominis and IBS. However, we hope that this antibody test will help in elucidating the controversy that surrounds the role of B. hominis as a pathogen at present.