Background: Vitamin D is suggested to play a role in airway and systemic inflammation in chronic obstructive pulmonary disease (COPD). Low serum 25-hydroxyvitamin D (25-OHD) levels have been associated with lower lung function and poorer exercise capacity in COPD. Objectives: The main purpose of this study was to explore whether the rate of decline in exercise capacity differs between COPD patients with and without vitamin D deficiency. Methods: A total of 166 subjects were selected from the Korean Obstructive Lung Disease cohort. Serum levels of 25-OHD below 20 or 10 ng/ml were defined as vitamin D deficiency or severe vitamin D deficiency, respectively. Exercise capacity was assessed by 6-min walk distance (6MWD). Results: No statistically significant difference was observed between 25-OHD ≥20 ng/ml group (n=35) and ＜20 ng/ml group (n=131). However, significant differences between 25-OHD ≥10 ng/ml group (n=121) and ＜10 ng/ml group (n=45) were observed for age, smoking status, lung function, and 6MWD. There was also a significant difference between the two groups in terms of the change in 6MWD over a 5-year period of follow-up (P＜0.001). Conclusions: The serum level of 25-OHD ＜10 ng/ml is clinically relevant to decreased exercise capacity in COPD. Severe vitamin D deficiency is associated with a greater decline in exercise capacity in patients with COPD. Intervention studies are warranted to determine whether vitamin D supplementation might be of benefit in COPD with severe vitamin D deficiency. Grant support from the Korea Healthcare Technology R&D Project (A102065).Background: Vitamin D is suggested to play a role in airway and systemic inflammation in chronic obstructive pulmonary disease (COPD). Low serum 25-hydroxyvitamin D (25-OHD) levels have been associated with lower lung function and poorer exercise capacity in COPD. Objectives: The main purpose of this study was to explore whether the rate of decline in exercise capacity differs between COPD patients with and without vitamin D deficiency. Methods: A total of 166 subjects were selected from the Korean Obstructive Lung Disease cohort. Serum levels of 25-OHD below 20 or 10 ng/ml were defined as vitamin D deficiency or severe vitamin D deficiency, respectively. Exercise capacity was assessed by 6-min walk distance (6MWD). Results: No statistically significant difference was observed between 25-OHD ≥20 ng/ml group (n=35) and ＜20 ng/ml group (n=131). However, significant differences between 25-OHD ≥10 ng/ml group (n=121) and ＜10 ng/ml group (n=45) were observed for age, smoking status, lung function, and 6MWD. There was also a significant difference between the two groups in terms of the change in 6MWD over a 5-year period of follow-up (P＜0.001). Conclusions: The serum level of 25-OHD ＜10 ng/ml is clinically relevant to decreased exercise capacity in COPD. Severe vitamin D deficiency is associated with a greater decline in exercise capacity in patients with COPD. Intervention studies are warranted to determine whether vitamin D supplementation might be of benefit in COPD with severe vitamin D deficiency. Grant support from the Korea Healthcare Technology R&D Project (A102065).