We have investigated small intestinal biopsies from children with coeliac disease, acute gastroenteritis, failure to thrive and giardiasis, to find out if a high intraepithelial lymphocyte count is a feature specific to coeliac disease, or whether it is always associated with partial or subtotal villous atrophy. The results indicate that the normal range for childrens' intraepithelial lymphocyte counts is similar to that for adults (around 6-40 lymphocytes per 100 epithelial cells); that counts are high in coeliac disease, but also in some children with giardiasis or with failure to thrive in whom the jejunal biopsy appears otherwise normal; and that intraepithelial lymphocyte counts are normal in acute gastroenteritis even when there is partial villous atrophy with increased lamina propria lymphoid cell infiltrate. Thus, this measurement of small intestinal lymphocyte infiltration may be of diagnostic value is differentiating the diarrhoea of food intolerance from infectious diarrhoeas in young children.