We administered both the rapid antigen test and real-time reverse transcriptase polymerase chain reaction (RT-PCR) test to 2154 patients in order to assess the diagnostic value of the rapid antigen test during Korean pandemics of H1N1 2009. The overall sensitivity was 70.0%, specificity was 97.5%, positive predictive value was 97.4%, and negative predictive value was 71.2%. The sensitivity fell and the negative predictive value decreased as the RT-PCR positive rate increased.