Respiratory syncytial virus (RSV) infection, a common lower respiratory infection in infants, is now recognized in the USA as a significant problem in elderly adults. RSV infection has rarely been reported in adults in Japan. Nasal samples from 77 patients with influenza-like illness (ILI) and negative for influenza in a rapid antigen detection kit were also tested by polymerase chain reaction (PCR) to identify RSV. A clinical trial was also conducted using a new antigen detection test kit for RSV based on immunochromatography. RSV was detected by nested RT-PCR in samples from nasal swabs of 10 patients--3 children and 5 adults--and nasal aspiration samples in 2 children. The frequency of RSV detection by nested RT-PCR in ILI patients with a negative response for influenza virus using the rapid detection kit was 27.3% (3/11) for children aged 0 to 1 year and 33.3% (2/6) for children aged 2-3 years. The frequency was 10% (1/10) for adults aged 30-39 years, 25% (1/4) for those aged 70-79 years, and 60% (3/5) for those aged 80-89 years. By month, the frequency was 25% (2/8) for December, 27.3% (6/22) for January, and 4.4% (2/45) for February. The main clinical symptoms of the 10 patients with RSV were: peak body temperature during the clinical course of 37.2-39.7 degrees C, cough, and rhinorrhea in 9. Stridor was observed in all five children, but not in the five adults. Clinical examination showed CRP to be 0.2-3.4 (mean 1.3) mg/dL and WBC to be 3070-8000 (mean 5584) /microL for nine patients. Lymphocytopenia was observed in the four adults from whom WBC fraction data was obtained. Chest X-ray was within normal limits. RSV was detected by the new rapid antigen detection kit in 9 of the 10 patients in whom RSV was detected by PCR, but not in any of the 67 patients in whom RSV was not detected. The diagnostic accuracy of the new antigen detection kit for RSV was thus excellent at 98.7% compared to PCR. RSV was detected from nasal swab specimens of a substantial number of elderly Japanese by PCR or the antigen detection kit.