Fifteen infants with acute viral bronchiolitis required mechanical ventilation. Infants were all aged less than 12 weeks and all had evidence of atelectasis or pneumonia on chest x-ray films. Respiratory syncytial virus was identified by immunofluorescence in 14 of 15 patients. Intubation and mechanical ventilation were initiated to reduce the work of breathing rather than to treat hypoxemia alone. Additional therapy included continuous positive airway pressure or positive end-expiratory pressure, sedation, aminophylline, diuretics, fluid restriction, and early feeding.