OBJECTIVES: The aim of our study was to examine pulmonary functions of patients from all stages of chronic obstructive pulmonarydisease (COPD) according to their smoking status.MATERIALS AND METHODS: A retrospective case-control study was carried out. Total of 148 patients were enrolled and divided intotwo groups in regards to their smoking status (quitters, n=68; non-quitters, n=80). Pulmonary function parameters, COPD assessment testscore, Fagerström Nicotine Addiction Questionnaire score, smoking history and status were obtained from the electronic hospital datasystem. Patients' admission and 12-month data were recorded.RESULTS: In non-quitters, the mean FEV1 values decreased from 2.32±1.14 to 2.24±1.12 (p<0.001). Particularly, in Stage-0, in the earlyhigh-risk group of COPD, the reduction in FEV1 was 90 mL, while the reduction was 70, 60, 40, and 40 mL in Stage-I, -II, -III, and -IV,respectively. In quitters, the mean FEV1 levels increased from 2.10±1.19 to 2.19±1.20 (p<0.001). For COPD patients overall, an averageincrease in FEV1 of 80–110 mL was observed. At the end of the 12 months follow-up, 17 (27.5%) of the non-quitters showed deterioration, and fie (7.3%) of the quitters showed improvement in COPD stage.CONCLUSION: FEV1 decline was accelerated in COPD patients who continued to smoke, whereas this decline was not prevented by inhaler treatments. The Global Initiative for Chronic Obstructive Lung Disease Stage-0 group, which is not included in the current guidelines,needs to be redefied. This group appears to be the most important group for implementing the smoking cessation and prevention strategy.