Magnus Kentson,1,2 Per Leanderson,3 Petra Jacobson,4,5 Hans Lennart Persson4,5 1Division of Medicine, Ryhov Hospital, Jönköping, Sweden; 2Department of Medical and Health Sciences (IMH), Linköping University, Linköping, Sweden; 3Department of Occupational and Environmental Medicine, Department of Clinical and Experimental Medicine (IKE), Linköping University, Linköping, Sweden; 4Department of Respiratory Medicine, 5Department of Medical and Health Sciences (IMH), Linköping University, Linköping, Sweden Background: The prevalence of individuals deficient in vitamin D (defined as a serum level of the stable metabolite 25(OH)D <50 nmol/L) is increasing in countries with low annual ultraviolet (UV) radiation and among individuals unable to perform outdoor activities, for example, COPD patients. Objective: To assess the role of vitamin D deficiency, independently of seasonal variation, the peak annual value of 25(OH)D was measured in subjects with advanced COPD ± long-term oxygen therapy (LTOT) and lung healthy control subjects. A method to grade the individual annual UV light exposure was designed and tested. Subjects and methods: Sixty-six Caucasians with advanced COPD (28 with LTOT) and 47 control subjects were included, and the levels of 25(OH)D were determined in late summer/early fall when the annual peak was assumed. Questionnaires about COPD symptoms, general health, lifestyle, dietary habits and QoL were used to collect data. Lung function tests and blood sampling were performed. Results: The peak annual 25(OH)D of COPD subjects was significantly lower than in the control subjects, but there was no significant difference between COPD patients with and without LTOT. Ongoing vitamin D supplementation was the single most important intervention to maintain 25(OH)D levels ≥50 nmol/L. Among vitamin D-deficient COPD subjects, 25(OH)D correlated positively with forced expiratory volume in 1 second as % predicted, Modified British Medical Research Council score, blood oxygenation, food portion size, Mediterranean Diet Score and Ultraviolet Score. Conclusion: Vitamin D deficiency was common among healthy individuals and COPD subjects. Peak annual 25(OH)D levels of COPD subjects correlated with clinically important outcomes. The present study emphasizes the need to routinely monitor vitamin D status among patients with advanced COPD and to consider to medicate those with vitamin D deficiency with vitamin D supplementation. Keywords: chronic obstructive pulmonary disease, inflammation, long-term oxygen therapy, quality of life, vitamin D, ultraviolet score