This study compared the incidence of febrile seizures (FS) reported prospectively up to 5 years of age, with the prevalence of FS by parental recall in the same cohort using the same questionnaire at 12 years of age. Both prospective and retrospective data were available for 807 children (389 males, 418 females). The number of children reported to have experienced FS in the prospective study was 57, and in the retrospective study was 45, yielding a cumulative incidence of 7.1 and 5.6% respectively. In the retrospective study there was an under-reporting of 19 children, over-reporting of eight children, and one child misreported by age at onset. Overall sensitivity of the retrospective approach was 65% and specificity was 99%. Positive predictive value was 82% and negative predictive value was 97%. Retrospective data underestimate the frequency of FS with high specificity but low sensitivity. Recall data suggest that some children with FS were not reported in the prospective data. These biases should be considered when evaluating the value of FS as a predictor of future health effects.