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Highlights • Particulate matter air pollution is common in military deployment environments of Iraq and Afghanistan. • Respiratory symptoms are frequently reported post-deployment, yet spirometry is frequently preserved. • Using the forced oscillation technique (FOT), 75% of veterans with preserved spirometry had distal airway dysfunction. • Veterans with distal airway dysfunction may represent at an at-risk population requiring closer surveillance. Abstract Purpose To evaluate the utility of the forced oscillation technique (FOT) among military veterans with preserved spirometry and chronic unexplained respiratory symptoms. Methods 178 veterans referred for evaluation of unexplained respiratory symptoms completed pulmonary function testing and FOT. Preserved spirometry was defined as FEV 1 /FVC, FEV 1 and FVC ≥ 5th percentile. Frequency dependence of resistance (R4-R20) and reactance area (AX) were assessed via FOT, and R4-R20 ≥ 20% and AX ≥ 95th percentile were considered abnormal. Results Spirometry was preserved in 71.3%, of whom 124 had acceptable FOT data. 93 of 124 (75.0%) veterans with preserved spirometry had one or more abnormal findings on FOT. Veterans with abnormal R4-R20 and/or AX had reduced FVC, FEV 1 , FEF 25-75 , and diffusing capacity (% predicted) in comparison to those with Normal FOT (p = 0.030 to p < 0.001). Conclusions In our referral sample, distal airway dysfunction in the presence of preserved spirometry appears common and may represent an at-risk group requiring closer surveillance. [ABSTRACT FROM AUTHOR]