The detection of collapsible airways, which may be a component of asthma and emphysema, has important therapeutic implications. We describe a patient with significant airways collapse contributing to his airflow limitation and discuss how collapsible airways can be assessed by the volume difference between what exits the lung as determined by a spirometer and the volume compressed as measured by the plethysmograph. More simply, a large volume difference between the slow and forced vital capacity (SVC-FVC), easily obtained from spirometry, may be used as a surrogate index of airway collapse.