Blastocystis hominis (B. hominis) is a protozoan that may inhabit the human gastrointestinal tract. In our study we reviewed the signs and symptoms of patients at Wilford Hall with stool specimens positive for B. hominis. These patients fell into four groups, HIV-positive adults, foreign nationals, children, and adults not known to be HIV positive. B. hominis caused an acute self-limited diarrheal illness, or chronic gastroenteritis with nausea, abdominal pain, and mild diarrhea. Metronidazole effectively relieved the symptoms and cleared the organism in some but not all patients.