A prospective study was carried out to investigate the epidemiology and clinical significance of Blastocystis hominis in the following groups of the population of the city of Salamanca (Spain): in children attending 11 day care centres and 7 primary schools, two fecal samples were obtained from each child, and in 1231 patients attending the Clinical Hospital. A B. hominis incidence of 5.3-10.3% was found in the day care centres and an incidence rate of 13.4-19.4% was found in the primary schools. All the cases were observed in asymptomatic children. The incidence of B. hominis was greater in children older than 3 years in the day care centres and in the 10-14 year-old group in the primary schools. A heavier parasitization was observed in the boys than in the girls and in the students of schools in areas of low socio-economic level. B. hominis was identified in 40 patients attending the Clinical Hospital (3.25% of all those studied). The maximum peak of incidence was found in subjects with ages between 10 and 14 years. A follow up study was performed on 18 patients parasitized exclusively by B. hominis; 7 of these were considered to have acute gastroenteritis and one chronic gastroenteritis associated with the protozoan. No statistically significant association was observed between the number of B. hominis cells and the presence of diarrhoea. Our results show that despite the high number of asymptomatic carriers of B. hominis in the juvenile population, this protozoan may be, on other occasions, responsible for gastrointestinal symptoms.