We evaluated influenza vaccination status as determined by self-report and a regional, real-time immunization registry during two influenza seasons when subjects were enrolled in a study to estimate vaccine effectiveness. We enrolled 2907 patients during the two consecutive seasons. The sensitivity and specificity of self-reported influenza vaccination when compared to immunization registry records were 95% and 90%, respectively. The positive predictive value of self-reported vaccination was 89% and negative predictive value was 96%. In our study population, self-reported influenza vaccine status was a sensitive and fairly specific indicator of actual vaccine status. Misclassification was more common among young children.