During a winter epidemic, 87 infants were admitted to Necker-Enfants-Malades hospital with a severe respiratory syncitial virus (RSV) infection. These infants fell into two groups: 37 infants without any medical history and 50 showing an underlying pathology (immune deficiencies, heart disease, CNS disorders, digestive malformations, allergic manifestations). Of the 37 infants with no medical history, most were below the age of 6 months and the RSV infection was manifested clinically by bronchiolitis or bronchitis. Most of the infants in the other group were more than 6 months of age and presented mostly with pneumonia or bronchiolitis. A respiratory distress syndrome was observed in 17 of the 87 infants, and virtually all of them were younger than 6 months. No significant difference was observed between the two groups with regards to the incidence of respiratory distress. Pulmonary infections complicating the course of the illness, most often due to commensal flora bacteria of the upper respiratory tract, were observed in 19 infants but with no greater frequency in the group at risk. Direct detection of viral antigens in nasopharyngal secretions not only enabled rapid diagnosis in all the infants but also allowed antiviral therapy to be started rapidly. Antiviral treatment by ribavirine, administered over a period of 5 days in 20 mg/ml doses by aerosol was instituted in 10 patients whose course might have become serious. In 8 of these patients, disappearance of the virus from secretions and recovery occurred. Two patients in the group at risk died despite treatment, with one case being considered a true therapeutical failure since the virus was still present in nasopharyngeal secretions.(ABSTRACT TRUNCATED AT 250 WORDS)