BACKGROUND: The detection of collapsible airways has important therapeutic implications in chronic airway disease and bronchial asthma. The distinction of a purely collapsible airways disease from that of asthma is important because the treatment of the former may include the use of pursed lip breathing or nasal positive pressure ventilation whereas in the latter, pharmacologic approaches are used. One form of irreversible airflow limitation is collapsible airways, which has been shown to be a component of asthma or to emphysema, it 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 plethysmography. METHOD: To investigate whether volume difference between slow and forced vital capacity(SVC-FVC) by spirometry may be used as a surrogate index of airway collapse, we examined pulmonary function parameters before and after bronchodilator agent inhalation by spirometry and body plethysmography in 20 cases of patients with evidence of airflow limitation(chronic obstructive pulmonary disease 12 cases, stable bronchial asthma 7 cases, combined chronic obstructive pulmonary disease with asthma 1 case) and 20 cases of normal subjects without evidence of airflow limitation referred to the Pusan National University Hospital pulmonary function laboratory from January 1995 to July 1995 prospectively. RESULTS: 1) Average and standard deviation of age, height, weight of patients with airflow limitation was 58.3+/-7.24(yr), 166+/-8.0(cm), 59.0+/-9.9(kg) and those of normal subjects was 56.3+/-12.47(yr), 165.9+/-6.9(cm), 64.4+/-10.4(kg), respectively. The differences of physical characteristics of both group were not significant statistically and male to female ratio was 14:6 in both groups. 2) The difference between slow vital capacity and forced vital capacity was 395+/-317ml in patients group and 154+/-176ml in normal group and there was statistically significance between two groups(p<0.05). Sensitivity and specificity were most higher when the cut-off value was 208ml. 3) After bronchodilator inhalation, reversible airway obstructions were shown in 16 cases of patients group, 7 cases of control group(p<0.05) by spirometry or body plethysmography and the differences of slow vital capacity and forced vital capacity in bronchodilator response group and nonresponse group were 300.4+/-306ml, 144.7+/- 180ml and this difference was statistically significant. 4) The difference between slow vital capacity and forced vital capacity before bronchodilator inhalation was correlated with airway resistance before bronchodilator(r=0.307 p=0.05), and the difference between slow vital capacity and forced vital capacity after bronchodilator was correlated with difference between slow vital capacity and forced vital capacity(r=0.559 p=0.0002), thracic gas volume(r=0.488 p=0.002) before bronchodilator and airway resistance(r=0.583 p=0.0001), thoracic gas volume(r=0.375 p=0.0170) after bronchodilator, respectively. 5) The difference between slow vital capacity and forced vital capacity in smokers and nonsmokers was 267.5+/-303ml, 277.5+/-276ml, respectively and this difference did not reach statistical significance(p>0.05). CONCLUSION: The difference between slow vital capacity and forced vital capacity by spirometry may be useful for the detection of collapsible airway and may help decision making of therapeutic plans.