We carried out clinical and basic studies of the Directigen Lateral Flow RSV (Becton, Dickinson and Company, USA), a rapid test kit that detects respiratory syncytial virus (hereinafter referred to as "RSV") antigens based on immunochromatography. For the clinical study, 103 nasopharyngeal aspirates from patients with acute respiratory infections were used to evaluate the kit. Compared to the cell culture method, the Directigen Lateral Flow RSV showed a sensitivity of 100% (16/16) and a specificity of 94.3% (82/87), and an agreement rate of 95.1% (98/103). When compared to conventional testing kits, we found that the total agreement rate with the Directigen RS (Nippon Becton Dickinson and Company) was 88.3% (91/103) and with RSV TestPack (Dainabot Co., Ltd.) was 91.3% (94/103). The detection limit of the Directigen Lateral Flow RSV was 2 x 10(3) PFU/ml for both RSV subgroups A and B. In the crossreactivity test, only RSV was found positive. No other microorganisms were crossreactive. We also studied storage stability of nasopharyngeal aspirates and found that stability was not affected by storage at room and refrigerator temperatures for 14 days. Taken all together, the Directigen Lateral Flow RSV is useful for the diagnosis of RSV infection in a clinical setting because its performance is equivalent to conventional testing kits and is easy to use.