To determine a more timely acquisition of accurate results for influenza patients, a rapid diagnostic testing for influenza were studied on 877 pediatric patients performed during the 2002-2003 flu season in our hospital. Of these, 337 patients were finally diagnosed as influenza based on the test results and treated with antiviral agents, amantadine or oseltamivir. Ten (29%) of the 34 patients whose tests were negative within 12 hours after onset became positive over 12 hours after onset. On the other hand, diagnoses based on antigen tests over 12 hours after onset were reliable because all 13 patients first confirmed negative were unchanged when tested afterward. These 10 patients missed the opportunity to take antivirals early, which possibly caused them to have significantly longer (p = 0.0003) febrile duration and higher frequency of admission (p < 0.0001) than the 106 patients first confirmed positive within 12 hours after onset. Days from onset until starting antivirals (mean 1.4 days), the febrile duration (mean 2.7 days) and frequency of hospitalization (20.5%) of the 219 patients who tested positive over 12 hours after onset were significantly worse (p < 0.0001, p < 0.0001 and p = 0.0406, respectively) than those of patients testing positive within 12 hours after onset (mean 0.2 days, mean 1.7 days and 11.3%, respectively). The febrile duration (mean 2.3 days) of the patients confirmed positive even over 12 hours, but within 48 hours, of onset was tolerable but significantly longer (p < 0.0001) than that of patients confirmed positive within 12 hours after onset. The frequency (19.6%) of hospitalization of the patients confirmed positive even over 12 hours, but within 48 hours, of onset was not significantly different from that of patients confirmed positive within 12 hours after onset. These results suggested that over 12 hours but within 48 hours after onset of illness is the best period for the rapid diagnosis to correctly determine whether a patient should be treated with antiviral agents based on the result.