Objectives.Most vaccine safety data present only the postvaccination incidence of all adverse events rather than an esti- mate of attributable risk. This study sought to illustrate the difference between the 2 estimates with data from a hepati- tis B immunization program. Methods. The incidence of health problems occurring before and after each dose of hepatitisB vaccine in a cohort of 1130 children were compared. Results.Although 47.5% of all chil- dren reported an adverse event during the 4 weeks following each of the 3 doses, adverse events attributable to immuniza- tion occurred in only 10.6% of children. Conclusions. Postimmunization in- cidence systematically overestimates the risk of adverse events. Estimating actual attributable risk is necessary to avoid false beliefs regarding immunization. (Am J Public Health. 2001;91:313–315)