RESULTS: 1. A total of 126 patients with stable COPD were enrolled. 2. Stable COPD: Sputum eosinophilia (≥2.5%) were observed in 56.3%, 52.7%, 49.5% in stable COPD at first, second and the third visit, respectively. The median level of sputum eosinophil were 5.4%, 5.6%, 5.3% at each follow-up respectively. A weak negative correlation between sputum eosinophil and FEV1% pred was found at every visit (rs1=−0.312, P<0.01, rs2=−0.222, P<0.05, rs3=−0.444, P<0.01 respectively). Sputum eosinophil of mild patients was significant lower than that of moderate to severe patients [1.7(0.7-4.0)% vs 2.5(0.8-4.6)%, 5.0(3.8-7.9)%, 5.7(1.5-8.3)%, P <0.01]. FEV1, FEV1% pred of patients with sputum eosinophilia decreased significantly. In addition, Borg scores of dyspnea and fatigue of patients with sputum eosinophilia were higher than those of patients without sputum eosinophilia. 3. Exacerbation: 49 patients experienced exacerbation. Sputum eosinophilia was found in 49.0% patients. Patients showed a trend of increased of sputum eosinophil at exacerbation [1.1(0.6-1.5)% vs 1.4(0.8-2.4)%]. Patients with sputum eosinophilia showed a trend of decreased of FEV1% pred, FEV1/FVC [58.2(33.2-73.1)% vs 63.6(43.2-68.8)%, 51.9(40.6-59.8)% vs 51.0(46.3-60.6)%]. A higher positive rate of PCR test for respiratory virus was found in patients with sputum eosinophilia when compared with patients without sputum eosinophilia (62.5% vs 28%, P<0.01). The most common virus were rhinovirus. The level of sputum eosinophil of patients with viral infection was significantly higher than that of patients without viral infection [3.4(2.1-6.3)% vs 1.7(0.8-2.9)%, P<0.05].