Background - Air trapping (AT) is a marker of obstructive lung diseases (OLD). However, its significance in the setting of normal FEV 1 /FVC is unclear. Methods - Lung function results from 646 patients who underwent PFT for OLD assessment between 2013 and 2015 at the San Francisco VA Medical Center were retrospectively examined for association between FEV 1 /FVC and AT (RV/TLC by plethysmography). Separately, we prospectively examined the maximum oxygen uptake (max VO 2 ) of a cohort of 127 subjects with normal FEV 1 /FVC and FEV 1 but varying degrees of AT. Associations between FEV 1 /FVC, RV/TLC, and exercise capacity were then examined. Results - In the retrospective analysis of the patients9 PFT (age=65±11 years; smoking history=83%; FEV 1 =81±26% predicted), RV/TLC was inversely associated with FEV 1 /FVC. The association was moderate when FEV 1 /FVC was abnormal (r=-0.56, p 1 /FVC ( 1 /FVC also had elevated RV/TLC (122±22% predicted). In the prospective study of the cohort (age=55±10 years) with normal spirometry (FEV 1 /FVC=97±5% predicted; FEV 1 =104±14% predicted), max VO 2 was inversely associated with RV/TLC, but not with FEV 1 /FVC, in age and height-adjusted multivariate modeling (r 2 =0.32 [p 2 per 1% increase in RV/TLC [p=0.039]). Conclusions - Many patients with normal FEV 1 /FVC have abnormal AT. In these patients without overt OLD by standard FEV 1 /FVC criterion, AT is associated with reduced exercise capacity.