Aislamiento de Escherichia coli O157: H7 a partir de heces de niños con síndrome urémico hemolíticoWe present the first report in Costa Rica of isolation of 2 strains of Escherichia coli O157:H7 as the ethilogical agent of diarrhoea associated to the uremic hemolytic syndrome. They were 4 cases studied at the National Childrens Hospital of Costa Rica: they were coming from different areas of the country without epidemiological relation among them. We recommend special attention to this bacteria in the study of those cases with diarrhoea and that also suggest an uremic hemolytic syndrome.