Minako Sato,1 Shotaro Chubachi,1 Mamoru Sasaki,1 Mizuha Haraguchi,1 Naofumi Kameyama,1 Akihiro Tsutsumi,1 Saeko Takahashi,1 Hidetoshi Nakamura,1,2 Koichiro Asano,3 Tomoko Betsuyaku1 1Division of Pulmonary Medicine, Department of Medicine, Keio University School of Medicine, Tokyo, 2Department of Respiratory Medicine, Saitama Medical University, Saitama, 3Division of Pulmonary Medicine, Department of Medicine, Tokai University School of Medicine, Kanagawa, Japan Background: Patients with COPD might not report mild exacerbation. The frequency, risk factors, and impact of mild exacerbation on COPD status are unknown. Objectives: The present study was performed to compare features between mild exacerbation and moderate or severe exacerbation in Japanese patients with COPD. Patients and methods: An observational COPD cohort was designed at Keio University and affiliated hospitals to prospectively investigate the management of COPD comorbidities. This study analyzes data only from patients with COPD who had completed annual examinations and questionnaires over a period of 2years (n=311). Results: Among 59 patients with mild exacerbations during the first year, 32.2% also experienced only mild exacerbations in the second year. Among 60 patients with moderate or severe exacerbations during the first year, 40% also had the same severity of exacerbation during the second year. Findings of the COPD assessment test and the symptom component of the StGeorge’s Respiratory Questionnaire at steady state were worse in patients with mild exacerbations than in those who were exacerbation free during the 2-year study period, although the severity of the ratio of predicted forced expiratory volume in 1second did not differ between them. Severe airflow limitation (the ratio of predicted forced expiratory volume in 1second