The incidence of Blastocystis hominis in a healthy population was determined by fecal examination of 6,422 Japanese and 54 resident non-Japanese who visited the St. Luke's International Hospital Health Screening Center for a routine medical check-up during a one-year period. Of the enrolled subjects, 30 Japanese (0.5%) and four non-Japanese (7.4%) had B. hominis in their stools. These individuals were asymptomatic except for one who reported flatus and one who reported mild abdominal discomfort. Statistical analysis indicated that the prevalence in the Japanese was lower than in the non-Japanese, and lower than the prevalence reported for other countries. Colonoscopic observations on seven B. hominis-positive individuals did not reveal pathogenic intestinal lesions. Several months after the first examination, 23 of the B. hominis-positive individuals, including three non-Japanese, were re-examined. Although they had not been treated with anti-B. hominis drugs, 10 individuals were now B. hominis-negative (by stool examination) and eight were passing fewer organisms. The remaining five individuals were still discharging large numbers of B. hominis. These B. hominis-positive individuals had no reported symptoms despite passing numerous organisms. Therefore, it seems that infection with B. hominis rarely gives rise to clinical symptoms. In no instance was invasion of host tissues by the organisms detected.