We conducted a 4-year retrospective cohort study to better define the risk of haemolytic anaemia and haemolytic uraemic syndrome (HUS) in children following sporadic gastrointestinal infection with the O 157.H7 serotype of Escherichia coli. Of the 72 children infected with this organism, 9 (12.5%) developed haemolytic anaemia, 6 of whom had HUS. No child in a cohort of 72 age-matched controls with Campylobacter jejuni gastroenteritis developed haemolytic anaemia (P = 0.003). Females had a significantly greater risk of developing haemolytic anaemia after E. coli O 157. H7 infection than did males (8/29 females v. 1/43 males; P = 0.003). In a logistic regression model, female gender emerged as the only statistically significant risk factor for haemolytic anaemia (odds ratio 3.85; 95% confidence interval 1.24-12). These results are consistent with recent reports of a moderate increase in the risk of HUS for females.