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Blastocystis hominis, a protozoan organism, may be capable of causing human disease. A child with watery, blood-streaked stools was found on colonoscopy to have multiple superficial ulcerations of the colon. Microscope slides made from biopsies revealed the ulcerations contained numerous Blastocystis organisms as well as white blood cells indicative of inflammation. Salmonella, Shigella, Yersinia, Campylobacter, and Clostridium difficile were ruled out. Oral metronidazole effected a cure. The pathogenic capabilities of this parasite have been debated. It is frequently found in human stool samples where there is no evidence of disease, and where it is found in conjunction with disease, other possible causes have been found as well.