Enteropathogenic Escherichia coli (EPEC) infection is not generally thought to cause severe diarrhoea after the neonatal period. Patients admitted to Queen Elizabeth Hospital for Children over the three years (1984-7) with diarrhoea and EPEC infection were reviewed. Clinical details, features of small intestinal mucosa, and treatment were recorded in those who developed chronic diarrhoea with failure to thrive. Twenty six children with EPEC required hospital admission for diarrhoea and six of these (23%) developed chronic diarrhoea. In contrast only two (5%) of 42 with other serogroups of E coli (p less than 0.01) and 28 (4%) of 764 children without EPEC admitted with acute diarrhoea developed chronic symptoms (p less than 0.01). EPEC serogroups detected in the stool of the six children with chronic diarrhoea were 0128 in three, 0114 in two, and 0119 in one. The patients' clinical characteristics were: previous good health, no significant immunodeficiency, age 4-10 months, foreign travel (three of six), severe life threatening secretory diarrhoea from 0.5 to 1.5 1 per day (four of six), small intestinal enteropathy (five of six) three of whom showed mucosal adherent, non-invasive E coli of the same serotype as that in the stool, in association with microvillous loss and pedestal formation. All were treated with hypoallergenic feeds, two with parenteral nutrition, and three with parenteral antibiotics. All eventually recovered. EPEC infection is a common treatable cause of life threatening chronic diarrhoea in infancy.