One way to maintain confidence in vaccination programmes is to improve monitoring of immunisation safety. We studied active parental reporting of adverse events after a booster dose of diphtheria-tetanus toxoid (DT). 7193 children received the vaccine. Questionnaires were submitted by 84.2% of the parents, who reported reactions for 9.2% of the children. Four percent of events were classified as moderate/severe by interviews. Relative risk of redness and swelling reported was 0.24 (95% CI, 0.13-0.42) compared to a clinical trial, while it was 71.0 (44-114) compared to passive surveillance. Active surveillance by parental reports is a useful complement to passive surveillance of childhood immunisations to generate hypotheses for evaluation in controlled studies.