373 (59%) out of 636 faecal specimens obtained during the first 2 years of life of 72 Mexican children yielded adherent Escherichia coli (HEp-2 cells). Strains with localised adherence were significantly associated with acute non-bloody diarrhoea, whereas strains with aggregative adherence were significantly associated with persistent diarrhoea. Half the strains with localised adherence were not enteropathogenic E coli serotypes nor did they hybridise with an enteropathogenic E coli adherence factor DNA probe. All strains with localised adherence gave a positive fluorescent actin staining (FAS) assay, irrespective of serotype. One-third of children colonised by aggregative strains had bloody diarrhoea. Isolation of strains with diffuse adherence was not related to type or duration of diarrhoea but was generally associated with isolation of another pathogenic organism.