Antibodies to Shiga toxin (Stx) were measured in the sera of 49 children with Shigella dysenteriae serotype 1 infection, of whom 17 had haemolytic uraemic syndrome (HUS) and 32 had no complications (uncomplicated shigellosis, UCS). Children with HUS had lower levels of total IgG and IgM and lower IgM titres to Stx than those with UCS. The number of children with neutralising antibodies was similar in the two groups. Of the children with HUS, 11 had HUS on enrolment and six developed HUS subsequent to enrolment. Antibody titres in children who subsequently developed HUS were compared with those in children with UCS to assess whether differences in antibody titres occurred before the development of HUS. IgA titres to Stx were found to be higher in children who subsequently developed HUS than in those with UCS. However, logistic regression analysis revealed that titres of Stx antibodies in the serum were not significant risk factors for the development of HUS. Thus, although the levels of Stx antibodies were different in children with HUS, and higher IgA titres to Stx were identifiable in children who subsequently developed HUS compared with those with UCS, the relevance of these findings in the development of HUS remains to be elucidated.