Clinical features on admission of 60 infants with acute bronchiolitis were related to disease severity. Crackles and cyanosis (which are related to oxygen requirements during the hospital stay) most closely correlated with severity, which was assessed by arterial blood gas analysis and pulse oximetry. Respiratory rate on presentation did not predict severity. Transcutaneous haemoglobin oxygen saturation on admission, measured by pulse oximetry, was closely related to cyanosis and maximum oxygen requirements. The best method for initial assessment of bronchiolitis was pulse oximetry.