The first recognized outbreak of haemorrhagic colitis due to Escherichia coli O 157.H7 in the United Kingdom affected at least 24 persons living in East Anglia over a 2-week period. The illnesses were characterized by severe abdominal pain and bloody diarrhoea of short duration. Eleven patients were admitted to hospital and there was one death. Patients were mainly adult women who had not eaten out of the home in the 2 weeks before onset. Unlike previously reported outbreaks hamburgers were not the vehicle of infection, and a case-control study suggested that handling vegetables, and particularly potatoes, was the important risk factor.