Infection with respiratory syncytial virus (RSV) is an important cause of illness in infants often leading to hospital admission during the winter months. Diagnosis of RSV by direct detection of virus antigen has been implemented in a number of departments of clinical microbiology, and on this background a descriptive study of the seasonal incidence, sex and age distribution of RSV in two Danish counties was undertaken. A total of 5533 clinical episodes of respiratory infection were registered from October 1988 to September 1992 in the two counties and 1421 (26%) were positive for RSV. The incidence of hospital admission for RSV infection was 14 per 1000 below three years of age in Copenhagen County in the winter season 1991-1992. The seasonal peak of RSV infection was postponed about one month in Northern Jutland as compared to the capital area. Only 10 episodes (three positive) were from primary care, the remainder from hospitals and other institutions. RSV-testing was rarely requested in general practice. The study confirmed the importance of RSV infection in young children admitted with lower respiratory tract infection. We recommend the establishment of a surveillance system incorporating the local departments of clinical microbiology for warning of the yearly epidemics during which RSV infections may also be a risk outside the paediatric age span.