Purged stools from 389 patients were evaluated microscopically for the presence of Blastocystis hominis. A total of five or more B. hominis cells per 40X field were observed in 43 patients (11%), and B. hominis was the only intestinal parasite present in 23 (6%) of these patients. Of the 23 patients, 19 had symptoms which included abdominal discomfort (15 patients), anorexia (10 patients), diarrhea (9 patients), and flatus (9 patients). The remaining four patients were asymptomatic. The proportion of eosinophils in the peripheral blood ranged from 4 to 12% in 11 (58%) of the symptomatic patients. Absolute eosinophil counts were greater than 250/microliter in 8 patients and greater than 400/microliter in 5 patients. Eosinophilia was not observed in the remaining symptomatic or asymptomatic patients. This study supports the emerging concept of the role of B. hominis as an intestinal parasite causative of human disease.