Objective: To investigate the potential role of endothelial dysfunction as a pathogenic mechanism of cardiovascular events in COPD. Hypothesis: endothelial function is abnormal in patients with COPD, particularly during exacerbations of the disease. Design: prospective and controlled study performed in an university hospital. Population: we compare 4 groups of subjects, 44 patients with stable COPD, 35 patients with acute exacerbation of COPD, 10 smokers or former smokers with normal lung function and 10 never smokers matched by physical activity and BMI. Study variables: a) clinical variables: clinical information, physical examination, pulmonary function tests, ECG and sputum culture; b) Biological variables: number of Circulating Endothelial Progenitor Cells, vascular growth factors (vascular endothelial growht factor and erythropoietin), systemic inflammation (C-reactive protein, white blood cells), peripheral venous blood test (including hemogram, biochemical analysis with glycemia, cholesterol, LDLcol, HDLcol); and c) systemic vascular reactivity assessment.