Quantitative shedding patterns of respiratory syncytial virus in 40 infants hospitalized with acute disease of the lower respiratory tract were determined for elucidation of the pathophysiology of infection with the virus. Nasal wash specimens were collected on admission and daily thereafter and were tested for the presence and quantities of respiratory syncytial virus. The following pattern of shedding was observed. (1) The virus was shed for prolonged periods. For the first seven days of hospitalization, 92%-100% of the infants tested continued to shed virus. At discharge 87% were still shedding the virus. (2) Respiratory syncytial virus was present in high titer in the nasal secretions obtained at the time of admission. The mean titer in these samples was 5.0 log10 TCID50. (3) The titer of respiratory syncytial virus did not fall during the first few days of hospitalization, despite clinical improvement of the infants. Neither peak nor admission titers of virus could be correlated with age or with the severity of disease. However, the mean admission titer in patients with bronchiolitis appeared to be significantly higher than that in those with pneumonia.