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Background: The functional effects of abnormal diffusing capacity for carbon monoxide (DLCO) in ex-smokers without chronic obstructive pulmonary disease (COPD) are not well understood. Objective: We aimed to evaluate and compare well established clinical, physiological and emerging imaging measurements in ex-smokers with normal spirometry and abnormal DLCO with a group of ex-smokers with normal spirometry and DLCO and ex-smokers with Global Initiative for Chronic Obstructive Lung Disease (GOLD) stage I COPD. Methods We enrolled 38 ex-smokers and 15 subjects with stage I COPD who underwent spirometry, plethysmography, St George's Respiratory Questionnaire (SGRQ), 6 min Walk Test (6MWT), x-ray CT and hyperpolarised helium-3 (³He) MRI. The 6MWT distance (6MWD), SGRQ scores, ³He MRI apparent diffusion coefficients (ADC) and CT attenuation values below -950 HU (RA950) were evaluated. Results: Of 38 ex-smokers without COPD, 19 subjects had abnormal DLCO with significantly worse ADC ( p=0.01), 6MWD ( p=0.008) and SGRQ (p=0.01) but not RA950 ( p=0.53) compared with 19 ex-smokers with normal DLCO. Stage I COPD subjects showed significantly worse ADC (p=0.02), RA950 (p=0.0008) and 6MWD (p=0.005), but not SGRQ (p=0.59) compared with subjects with abnormal DLCO. There was a significant correlation for ³He ADC with SGRQ (r=0.34, p=0.02) and 6MWD (r=-0.51, p=0.0002). Conclusions: In ex-smokers with normal spirometry and CT but abnormal DLCO, there were significantly worse symptoms, 6MWD and ³He ADC compared with exsmokers with normal DLCO, providing evidence of the impact of mild or early stage emphysema and a better understanding of abnormal DLCO and hyperpolarised ³He MRI in ex-smokers without COPD. [ABSTRACT FROM AUTHOR]