['The parasites Dientamoeba fragilis and Blastocystis have been detected in feces from patients with irritable bowel syndrome (IBS), therefore these parasites may be involved in IBS pathogenesis. We proposed that a higher prevalence of the parasites in IBS subjects compared with asymptomatic controls would support such a mechanism. We aimed to determine the prevalence of these parasites in IBS subjects (cases) and controls and to identify risk factors associated with parasite carriage.', 'We performed a population-based, case-control study of an adult population from an internet-based research institute in Denmark. In January 2010, subjects completed a questionnaire based on the Rome III criteria for IBS and answered questions on factors associated with parasite carriage. Respondents (n\xa0= 483) were asked to submit fecal samples for parasite testing; samples were analyzed from 124 cases and 204 controls.', 'A greater proportion of controls than cases carried the parasites (50% vs 36%; P\xa0= .01). D fragilis was detected in a greater proportion of fecal samples from controls than cases (35% vs 23%; P\xa0= .03), as was Blastocystis (22% of controls vs 15% of cases; P\xa0= .09), and a greater percentage of controls carried more than 1 species of parasite (16% of controls vs 8% of cases; P\xa0= .05). D fragilis infection was associated with having children 5 to 18 years old in the household and Blastocystis infection was associated with high income (≥600,000 Danish Kroner/y, approximately $100,000 US dollars/y), no animals in the household, and drinking bottled water.'] Is the prevalence of intestinal parasites greater among individuals with irritable bowel syndrome : a population-based case-control study?