Over a 3-week period, 20 of 34 (59%) infants in a newborn nursery developed nosocomial viral respiratory tract disease. Either respiratory syncytial virus (seven infants) or parainfluenza virus type 3 (five) or both (two) were demonstrated in respiratory secretions from 14 of the 20 symptomatic patients. Symptoms in the 20 infants included rhinitis (15 infants), cough (14), apnea (eight), pulmonary infiltrates (seven), and fever (six). There were no differences in symptoms between children infected with respiratory syncytial virus alone, with parainfluenza virus alone, or with both viruses concurrently. Patients were clustered in the nursery by agent: infants with the same virus tended to share contiguous bed spaces, supporting the concept that parainfluenza virus as well as respiratory syncytial virus can be transmitted from patient to patient. In addition to this risk for contiguous bed spaces, the presence of a nasogastric tube was associated with risk of illness (P less than 0.05). In the presence of a nursery outbreak of respiratory tract disease, more than one virus may circulate concurrently, and an individual patient may be infected simultaneously by more than one virus.