Vitamin D is well known for its role in bone health and calcium homeostasis. However, emerging evidence suggests that vitamin D may also have an influence on other health issues such as type 2 diabetes, cardiovascular disease, cancer, autoimmune diseases and respiratory function(1,2). Chronic obstructive pulmonary disease (COPD) is characterised by airflow limitation and is associated with an abnormal inflammatory response(3). It has been hypothesised that vitamin D deficiency may enhance chronic airway and systemic inflammation and increase the risk of infectious exacerbations(1) in COPD. Vitamin D status in this population is, therefore, of interest. The aim of the present study was to examine the prevalence of sub-optimal vitamin D status in a sample of patients with moderate to severe COPD and to compare the vitamin D status of a sub-sample of this patient group with healthy age-, sex-, and season-matched controls.The serum 25-hydroxyvitamin D (25-OHD) levels of 81 Northern Irish patients with moderate to severe COPD (classified according to Global Initiative for Chronic Obstructive Lung Disease (GOLD) guidelines(3)) were measured using Ultra Performance Liquid Chromatography followed by tandem mass spectrometry (UPLC/MS/MS). Twenty-two COPD patients were matched for age-, sex-, and season of blood sample collection with healthy participants recruited from Northern Ireland at the same time as the COPD cohort.The mean vitamin D status for male and female COPD patients was 35.3 (SD 23.3) nmol/l and 38.5 (SD 49.6) nmol/l, respectively (P=0.701; Independent samples t-test). There was no significant difference in mean vitamin D status between those with moderate (32.3 (SD 20.2) nmol/l) compared to severe COPD (39.8 (SD 44.8) nmol/l) (P=0.372; Independent samples t-test). Season of blood sample collection had no significant effect on vitamin D status in this group of COPD patients (P=0.116; analysis of variance). Overall, almost half of the COPD patients (45.7%) were vitamin D deficient (75nmol/l). The vitamin D status of patients with moderate to severe COPD (n=22) was significantly lower compared with healthy age-, sex-, and season-matched controls (n=22) (36.7 (SD 36.8) nmol/l versus 52.5 (SD 23.2) nmol/l, respectively; P=0.003; Independent samples t-test). Suboptimal vitamin D status was prevalent in this sample of patients with moderate to severe COPD. Furthermore, patients with moderate to severe COPD had significantly lower vitamin D concentrations compared to healthy age-, sex-, and season-matched controls. The potential role of vitamin D supplementation in the prevention or management of COPD is worthy of further investigation.1.Janssens W, Lehouck A, Carremans C et al. (2009) Am J Resp Crit Care Med 179, 630-636.2.Gilbert CR, Arum SM & Smith CM. (2009) J Can Thorac Soc 16, 75-80.3.Global Initiative for Chronic Obstructive Lung Disease (GOLD) guidelines. (2001) NHLBI/WHO Workshop Report.