Two hundred thirteen infants younger than 13 months with bronchiolitis were prospectively followed up to identify the historical, physical, and laboratory clues at initial emergency department evaluation that would help to predict disease severity. Based on their total course of illness, the patients were classified as having mild (139 patients) or severe (74 patients) disease, and the initial emergency department evaluation findings of these two groups were compared. Six independent clinical and laboratory findings were identified that were strongly associated with more severe illness: (1) "ill" or "toxic" general appearance; (2) oxygen saturation less than 95%, as determined by pulse oximetry; (3) gestational age, younger than 34 weeks; (4) respiratory rate, 70/min or greater; (5) atelectasis on a chest roentgenogram; and (6) age, younger than 3 months. The infant's oxygen saturation as determined by pulse oximetry was the single best objective predictor of more severe disease.