Enterohemorrhagic Escherichia coli (EHEC) are associated with hemorrhagic colitis and hemolytic uremic syndrome. These illnesses are typically seen in young children, but are rare before 6 mo of age. The cause of this age restriction is unclear. Because bacterial adherence to intestinal mucosa is considered a critical initial event in pathogenesis, we studied the ontogeny of the adherence of EHEC (0157: H7 and other serotypes) isolated from children with diarrhea, hemorrhagic colitis, or hemolytic uremic syndrome. Adherence was quantitatively determined by incubating radiolabeled bacteria with viable rabbit intestinal cells, which were prepared by treating loops of distal ileom and proximal colon with EOTA, DTT, and citrate. Cells obtained from animals of different ages were studied simultaneously. The adherence of the various EHEC strains varied significantly. A non-0157: H7 E. coli strain 43–12 bound best (35 and 32 bacteria/cell to ileal and colonic cells, respectively) with 48–60% inhibition by D-mannose and α-methyl mannoside (p< 0.01) and 20–28% inhibition by L-fucose (p< 0.05), but no significant inhibition by other carbohydrates. Analysis of variance and polynomial regression showed that postnatal age significantly affected the adherence to ileal and colonic cells. Adherence during the 1st wk of life was 13–19% of that in the adult animal; it increased gradually, reaching the adult level at about 4 wk of age. Our study shows that postnatal age affects the adherence of EHEC to intestinal cells. These findings are compatible with postnatal development of gut receptors and may be relevant to the age-related risk of EHEC disease in children.