Background: Vitamin D deficiency has been associated with chronic disorders including chronic obstructive pulmonary disease (COPD) but the relationships with inflammation, exacerbations and disease progression remain unclear. Methods: In this monocentric cross-sectional observational study we analyzed the disease status, systemic inflammation, prior exacerbation frequency and loss in lung function in relation to serum 25-hydroxyvitamin D (25-OHD) levels in a cohort of 94 patients with COPD. Serum 25-OHD, C-reactive protein, interleukin-6 and tumor necrosis factor-α were quantified. Exacerbation frequencies and sunlight exposure were assessed. These parameters were analyzed in correlation to the current forced expiratory volume in 1 s (FEV 1 ), the individual average 3-year FEV 1 decline and the Global Initiative for Chronic Obstructive Lung Disease (GOLD) stage. Results:We observed fair correlation between serum 25-OHD and the current FEV 1 (r=0.38, P<0.001). Furthermore, mean serum 25-OHD was significantly altered between patients of GOLD stages I-IV (P=0.013). There was weak negative correlation of 25-OHD and the average annual change of the FEV 1 (r=−0.26, P<0.05). Furthermore, we observed fair negative correlation between 25-OHD and C-reactive protein (r=−0.32, P<0.01) as well as weak negative correlation with interleukin-6 (r=−0.23, P<0.05). While the exacerbation frequency significantly differed between GOLD stages (P=0.04), there was no direct association between exacerbations and 25-OHD levels. Conclusion:Our data confirm frequent vitamin D deficiency in COPD and point out correlations between 25-OHD levels, systemic inflammation, disease severity and progression.