Seventeen out of 54 preschool children at a day nursery became infected with Escherichia coli O 157. The nursery outbreak formed part of a wider community outbreak, whose source was not identified. Epidemiological investigation suggested that the nursery outbreak resulted from person to person spread. Children with diarrhoea or a positive faecal specimen were excluded until their symptoms had resolved and one negative faecal specimen had been obtained. Children who were asymptomatic throughout were not excluded. This policy appeared to be effective in preventing further spread of infection within the nursery.