Correlation of chest radiographic appearances with concurrent fibreoptic bronchoscopy in patients observed while in intensive care, and with lobar or multilobar atelectasis due to retained secretions, suggests that three levels of air bronchogram can be identified: Proximal: air is visible only to the level of the main bronchus, bronchus intermedius or lobar bronchi. Intermediate: air is visible down to the segmental bronchi. Distal: air is visible down to the subsegmental bronchi or beyond. Proximal or intermediate air bronchograms signify accumulation of secretions in central bronchi and indicate the need for fibreoptic bronchoscopy to aspirate secretions: a distal air bronchogram signifies peripheral bronchial obstruction thus precluding effective fibreoptic bronchoscopy; chest physiotherapy is then a more appropriate treatment. The proximal air bronchogram may change in extent or definition-sometimes rapidly; a receding or an increasingly poorly defined proximal air bronchogram are danger signals of centrally accumulating secretions which may lead to respiratory arrest.