Pulmonary hypertension is a common complication in chronic obstructive pulmonary disease (COPD). Its presence and severity is closely related to disease prognosis. Remodelling of pulmonary vessels is the principal causative factor of pulmonary hypertension in COPD. In advanced COPD, pulmonary vascular remodelling is related to the severity of arterial hypoxaemia. However, recent studies have shown that structural abnormalities and alterations of vascular function are also apparent in patients with mild COPD who do not have hypoxaemia and even in smokers with normal lung function. Pulmonary endothelium plays a crucial role in the regulation of vascular tone and cell growth of the vessel wall. Alterations of endothelial function in pulmonary arteries are apparent at the early stages of chronic obstructive pulmonary disease evolution. Potential mechanisms of endothelial damage at these initial stages include the effects of cigarette smoke components and inflammatory changes. The resultant alteration of pulmonary endothelium by these factors might predispose patients with mild chronic obstructive pulmonary disease to further vascular damage by additional factors, such as hypoxaemia, ultimately leading to pulmonary hypertension.