School age children carry the heaviest burden of morbidity due to intestinal parasitic infections. Our objective was to determine the prevalence of these infections in primary school children living in Tehran and their association with socio-economic factors and hygienic habits. In September 1998, a total of 19,213 subjects were invited to participate. Data on health and socio-economic status and health-related behaviours, collected via questionnaires, physical examinations and stool sample analyses, were available for 19,209 persons, with a participation rate greater than 99.99%. All participants were subjected to three methods of microscopic examinations on the stool sample and an adhesive cellophane tape slide evaluation. The prevalence rate of intestinal parasitic infection among the students was 18.4%. Coinfection with two or three parasites was seen in 2%. With increase in educational level of parents (especially mothers), the infection rate of children was decreased. Girls showed a significantly higher positive rate than boys. Using piped water, correct method of washing vegetables and increase in the economic score of the family, resulted in a decrease in the infection rate. According to the results, low level of education and consequently poor socio-economic and hygienic condition of families appear to be powerful determinants of infection.