The morbidity of intestinal parasitic infections is greatest among children of school age and may have an adverse effect on growth. In the present, cross-sectional study, the association between previously undiagnosed intestinal parasitic infections and growth was assessed in 19,209 children attending elementary schools in Tehran. The physical growth of these children was investigated by recording body weights, heights and weight-for-age Z scores. Faecal samples were collected and each checked for intestinal parasites using four methods (the microscopical examination of a smear stained with Lugol's iodine, a smear prepared by formol-ether concentration, a wet smear in physiological normal saline, and a strip of adhesive tape that had been pressed against the subject's peri-anal region). The association between intestinal parasitic infection and growth was explored using multivariable models adjusted for the influence of age and polyparasitism. The prevalence of infection with any intestinal parasite was 18.4%. The prevalences of stunting (3.8% v. 2.8%), wasting (22.7% v. 20.4%) and the combination of stunting and wasting (3.7% v. 2.8%) were all significantly higher in the infected children than in the uninfected (P<0.01 for each). Although at least nine species of parasite were detected, only two, Giardia lamblia and Enterobius vermicularis, were each significantly associated with low height for age (stunting) and low weight for height (wasting). There were no such reductions with Ascaris lumbricodes, hookworm, Blastocystis hominis, Hymenolepis nana or Entamoeba coli infection. The data indicate fairly high prevalences of previously undiagnosed intestinal parasitic infections and highlight important interactions between nutrition and some of the parasites detected. School health programmes aimed at reducing the prevalences of parasitic infections (particularly giardiasis and enterobiasis) in schoolchildren, which may well have beneficial effects on growth and educational outcome, are clearly needed in Tehran.