A prospective observational evaluation of treatment outcomes of patients with chronic obstructive pulmonary disease and peripheral blood eosinophilia Introduction: This study aimed to compare two groups of patients with Chronic Obstructive Lung Disease (COPD) and blood eosinophil values of ≥300 cells/ µL. The patients were followed up for one year in two groups, one receiving inhaled corticosteroids (ICS) and the other not receiving ICS in terms of exacerbation rates, the incidence of pneumonia, mortality rates, FEV1, FVC and FEF25-75 changes, the COPD Assessment Test (CAT) and modified Medical Research Council (mMRC) dyspnea scores. Materials and Methods: Stable patients, who presented to the University Medical Faculty Chest diseases outpatient clinic and diagnosed with COPD according to pulmonary function test results and GOLD stages 2-4 according to the 2017 GOLD criteria and with blood eosinophil counts of ≥ 300 cells/ µL were included in the study. Results: 85 patients were included in the study, of which 76.5% (n= 65) were males. There were no significant differences between the 38 patients receiving ICS and the 47 patients not receiving ICS regarding baseline FEV1, FVC, FEF25- 75 absolute values and percentages, and FEV1, FVC values controlled at month six and month 12, and FEF25-75 absolute values and percentages at month six (p> 0.05). However, the difference between the baseline and month 12 absolute values and percentages of FEF25-75 was statistically significant, and the decrease from baseline was greater in the group receiving ICS (p< 0.05). The difference in CAT and mMRC scores were statistically significant at month six and month nine in favor of the group receiving ICS (p< 0.05). The median number of moderate and severe exacerbations were both one (min: 0, max:3) in the group receiving ICS (n= 38) and in the one not receiving ICS (n= 47). The difference of exacerbation rates in both groups were not statistically significant (p> 0.05). Conclusion: In this study, it was determined that exacerbation rates of eosinophilic COPD patients did not differ depending on the use of ICS in a one-year period. Nevertheless, the 12-month FEF25-75 values of patients not using ICS were lower to a lesser extent, while the CAT and mMRC scores improved in favor of ICS users at different months in the one-year follow-up