*Data are presented as mean±SD; all parameters recorded at baseline; health/disease state based on screening and medical history and smoking status based on self-reported history and urine nicotine metabolite levels (detailed in online supplementary methods); non-smokers were comparable with all healthy smokers and all COPD smokers in ethnicity, BMI and all pulmonary function (p>0.1, all comparisons), except for FEV1 and DLCO that were lower in all COPD smokers (p<10⁻⁷, both comparisons), and FEV1/FVC, that was lower in all healthy smokers and, by definition, in all COPD smokers (p<0.02, both comparisons). Non-smokers were younger than all healthy smokers and all COPD smokers (p<0.002, both comparisons), and there were less female COPD smokers than female non-smokers (p<0.0002). There were more COPD smokers with emphysema compared with non-smokers (p<10⁻⁴); All healthy smokers were comparable with all COPD smokers in ethnicity, age, BMI, all smoking history parameters (p>0.3, all comparisons), except for pack-year that was lower in all healthy smokers (p<10⁻³). FVC and TLC were comparable (p>0.07, both comparisons), but FEV1, DLCO and, by definition, FEV1/FVC were lower in all COPD smokers (p<10⁻¹², all comparisons). There were fewer females among all COPD smokers than among all healthy smokers (p<0.04). There were more COPD smokers with emphysema compared with healthy smokers (p<10⁻⁴).

†Healthy smokers who continued smoking had urine cotinine ≥104 ng/mL (see online supplementary methods for details) at baseline, 3,6 and 12 months. Healthy smokers who quit had undetectable urine nicotine and cotinine levels at 3, 6 and 12 months. The healthy smokers who continued smoking were comparable with those who quit in age, ethnicity, all smoking history (p>0.3, all comparisons), except for pack per day that was lower in those who quit (p<0.03), and comparable in all pulmonary function (p>0.1). There were more females, and the BMI was higher in the healthy smokers who quit group (p<0.04, both comparisons).

‡Gold stage defined by GOLD criteria1; see online supplementary methods for details of subjects on medications; several of those treated were on multiple classes of medications; COPD smokers who continued smoking had urine cotinine ≥104 ng/mL at baseline, 3, 6 and 12 months; COPD smokers who quit had undetectable urine nicotine and cotinine levels at 3, 6 and 12 months; The COPD smokers who continued smoking were comparable with those who quit in age, gender, ethnicity, all smoking history and all pulmonary functions (p>0.3, all comparisons), except for DLCO that was lower in the COPD who continued smoking compared with those who quit (p<0.03). The BMI was lower in the COPD who continued smoking versus those who quit (p<0.002). There was no difference in the number of subjects with emphysema between the COPD smokers who quit smoking and those who continued smoking (p>0.6).

§B, black, W, white, O, other.

¶Chest high-resolution CT (HRCT); % emphysema at −950 Hounsfield Units (HU); emphysema defined as >5% lung volume; see online supplementary methods for details.

**Pulmonary function testing parameters are given as % of predicted value with the exception of FEV1/FVC, which is reported as % observed. BMI, body mass index; DLCO, diffusing capacity; GOLD; Global Initiative for Chronic Obstructive Lung Disease; TLC, total lung capacity.