The prevalence of Blastocystis hominis in stool specimens of individuals with gastrointestinal symptoms was evaluated to study a possible link between the protozoan and the irritable bowel syndrome. According to the Rome diagnostic criteria, 388 patients were evaluated. Altogether, 81 patients were classified as affected by irritable bowel syndrome. Blastocystis hominis was recovered from the stools of 38 subjects, 15 of whom belonged to the group with irritable bowel syndrome (P = 0.006). In addition, patients with irritable bowel syndrome were significantly more likely to have five or more Blastocystis hominis organisms per field (P = 0.031). In conclusion, there was a set of patients with irritable bowel syndrome in whom the presence of Blastocystis hominis may not be incidental.