A total of 19,252 stool specimens from 12,136 patients were examined by direct microscopy and the ethyl acetate-Formalin concentration method during the last 2 years. All liquid specimens and those in which parasite identification was difficult or equivocal were also examined in trichrome-stained preparations. A total of 3,070 intestinal parasites were seen in 2,889 patients. Blastocystis hominis was found in fecal material from 647 patients (17.5%). A total of 132 cases (25.6%) were observed to be in association with other enteric pathogens. B. hominis in large numbers was present as the only parasite or with other commensals in 515 specimens from patients (79.6%). Of these patients, 239 (46.4%) had symptoms, the most common being abdominal pain (87.9%), constipation (32.2%), diarrhea (23.4%), alternating diarrhea and constipation (14.5%), vomiting (12.5%), and fatigue (10.5%). Forty-three (18%) of the patients were treated with metronidazole (0.5 to 1.0 g/day) because of recurrent symptoms and the presence of large numbers of B. hominis cells in repeated stool specimens. After 7 to 10 days of treatment, all patients became asymptomatic with negative stools on follow-up examinations for B. hominis.