Background Airflow limitation (AFL) deterioration increases residual volume (RV), functional residual capacity (FRC), and total lung capacity (TLC) through air trapping and hyperinflation. Since TLC is the gold standard to confirm or exclude the presence of restrictive ventilatory defect when vital capacity is decreased, interpretation of pulmonary function test (PFT) would be different as TLC changes. We compared the changes in lung volume between mixed and obstructive ventilatory defect in PFT when AFL deteriorates. Method Among patients who underwent spirometry and lung volume measurement simultaneously more than twice at Asan Medical Center for 3 years and Ulsan University Hospital for 10 years, MO and OO groups were defined as obstructive groups with an increase in RV by ≥300ml, those who were originally interpreted as having mixed (FEV1/FVC<0.7 and TLC<80% predicted) and obstructive (FEV1/FVC<0.7 and TLC≥80% predicted) ventilatory defects. We analyzed PFT Results of 31 pairs of two groups who were propensity score matched (1:1) for age, height, weight and forced expiratory volume in one second (FEV1). Results When AFL deteriorated, RV, FRC and TLC were increased in both groups but were significantly lower in MO group than in OO group (99.8% vs. 129.9%; 93.8% vs. 122.6%; 86.0% vs. 103.4%, respectively, p<0.001 by independent t-test). In particular, the extent of increase in FRC and TLC with RV increase was differentially less in the MO group than in the OO group (P for interaction<0.01). Through receiver operating curve analysis, the MO group could be distinguished from the OO group using the best cut-off level (≤63.3%) of FRC/TLC ratio (sensitivity=54.8%, specificity=77.4%, p=0.027). Conclusion With AFL deterioration, the extent of increase in FRC and TLC with RV increase is less in mixed ventilatory defect than in obstructive ventilatory defect, and FRC/TLC ratio is a useful parameter to distinguish between the two ventilatory defects.