We retrospectively examined the hospital course of 12 patients with exacerbated inflammatory bowel disease (IBD), who also had stool specimens positive for Blastocystis hominis to determine the effect of B. hominis on their disease. Bloody bowel movements were common with ulcerative colitis patients and watery diarrhea with Crohn's disease; other findings included abdominal pain, fever, nausea, and vomiting. All patients responded favorably to medical therapy. Three responded to treatment with corticosteroids alone, and one patient improved with bowel rest without medications. Five patients failed to improve on metronidazole; four of them responded to a subsequent course of corticosteroids, whereas the fifth patient became asymptomatic after erythromycin treatment for concomitant Campylobacter jejuni. Finally, three patients responded to treatment with metronidazole alone, which is known to eradicate B. hominis but may also have a beneficial effect on IBD. In no case did corticosteroid treatment worsen the condition. Our findings indicate that B. hominis is not a significant pathogen in IBD and treatment must be directed toward the underlying illness.