Infants hospitalized with bronchiolitis are frequently monitored with a pulse oximeter. However, there is little consensus on an acceptable lower limit of oxygenation. No previous studies have examined how the use of pulse oximetry and supplemental oxygen therapy affects length of stay. To determine the extent to which bronchiolitis hospitalizations are prolonged by a perceived need for supplemental oxygen based on pulse oximetry readings. Design and Patients Retrospective case series of subjects younger than 2 years who were hospitalized with bronchiolitis at an academic medical center. Two investigators independently reviewed the hospitalization records of 73 infants and determined at what point an infant met all discharge criteria except oxygenation. We then calculated the extent to which hospitalizations were prolonged by a perceived need for supplemental oxygen therapy based on pulse oximetry readings alone. Sixty-two infants met inclusion criteria. There was high interrater reliability in determining whether hospitalizations were prolonged (kappa = 0.75). In 16 (26%) of 62 patients (95% confidence interval, 15%-37%), the hospitalization was prolonged because of oxygenation concerns. Length of stay was prolonged an average of 1.6 days (range, 1.1-2.0 days) per hospitalization for these 16 patients, or 0.4 day (range, 0.2-0.6 day) per hospitalization for all 62 patients. Hospitalizations of some infants with bronchiolitis are prolonged by a perceived need for supplemental oxygen therapy based on pulse oximetry readings. Further investigation into outcomes of different levels and durations of oxygen desaturation is needed and would have the potential to reduce practice variability and shorten the length of stay.