Progressive decline in lung function is the hallmark of chronic obstructive pulmonary disease (COPD). We aimed to assess the rate of decline in forced expiratory volume in 1 second (FEV1) in patients from a community cohort database in Korea. 5,865 subjects aged 40-69 years from the Ansung-Ansan cohort database I-III (2001-2006) were included in this study. We assessed the annual rate of decline in FEV1 over time in relation to smoking status, patient sex, and presence or absence of pre-bronchodilator airflow limitation using a generalized additive mixed model. The mean follow-up duration was 3.8 years. The annual mean decline in FEV1 in the entire cohort was significantly more rapid for men than women (31.3 mL vs 27.0 mL, P = 0.003). Among men without pre-bronchodilator airflow limitation, annual mean declines in FEV1 were 31.5, 35.5, and 40.1 mL for never smokers, former smokers (P = 0.09 vs. never smokers), and current smokers (P < 0.001 vs. never smokers), respectively; and 23.4, 19.7, and 33.9 mL, respectively, for men with pre-bronchodilator airflow limitation. Thus, among Korean males, smoking accelerates lung function decline over time whereas smoking cessation slows the rate of FEV1 decline regardless of pre-bronchodilator airflow limitation. This underscores the importance of smoking cessation in Koreans.