Surveys of childhood vaccinations are often highly inaccurate, due to parental misreporting, The authors conducted 3 experiments to examine the source of the inaccuracies. In Exp 1, parents were provided with memory aids; these aids did little to improve reporting accuracy. Two further experiments asked whether parents forgot what they knew about their children's vaccinations, or whether they never knew the information. Exp 2 surveyed parents both immediately and 10 wks after their child's medical visit. Accuracy was only slightly better than chance immediately afterwards; 10 wks later performance had not changed significantly. Exp 3 compared reports in both recall and recognition conditions. Although the recognition condition lowered the response burden on parents it did not produce more accurate reports. It is concluded that low levels of accuracy in parental reports on vaccinations appear to reflect poor initial encoding rather than retrieval failure. (PsycINFO Database Record (c) 2007 APA, all rights reserved)