We evaluated a flow-through immunoassay for rapid detection of influenza A and B viral antigens, RapidTesta FLU AB (Daiichi Pure Chemicals Co., Ltd., Tokyo, Japan), by using 507 specimens collected from patients with influenza-like symptoms during the 2002/2003 influenza season in Japan. The specimens consisted of 239 nasal swabs and 268 nasal aspirates; 374 specimens were collected from pediatric patients (under 16 years of age) and 133 from adult patients. RapidTesta FLU AB was compared with cell culture and nested reverse transcription-polymerase chain reaction (RT-PCR). Cell culture detected influenza virus from 66.7% of the 507 specimens (influenza AH3: 44.0%, B: 22.7%). For nasal swabs, it had a sensitivity of 81.9% (77/94), a specificity of 97.9% (142/145) and an efficiency of 91.6% (219/239) for influenza A virus as well as a sensitivity of 80.0% (52/65), a specificity of 98.3% (171/174) and an efficiency of 93.3% (223/239) for influenza B. For nasal aspirates, RapidTesta FLU AB had a sensitivity of 83.2% (109/131), a specificity of 98.5% (135/137) and an efficiency of 91.0% (244/268) for influenza A as well as a sensitivity of 82.7% (43/52), a specificity of 97.7% (211/216) and an efficiency of 94.8% (254/268) for influenza B. RapidTesta FLU AB is characterized by high specificity, low false positive rate, and 10-minute detection of influenza virus. These advantages suggest that RapidTesta FLU AB is a useful kit to assist physicians in making a diagnosis of influenza on candidates for antiviral therapy.