The chronic hypoxemia seems to play an important role in the pathogenesis of hemodynamic disorders in chronic obstructive pulmonary disease (COPD). The aim of the study was to follow-up the changes in pulmonary hemodynamics in COPD with mild respiratory failure. 78 COPD patients with Ist and IId degree of respiratory failure were investigated. The following investigations were carried out: two--dimensional echocardiography, spirometry, arterial blood gases and acid-base balance. In 18 of COPD patients a right heart catheterization was performed and the following indices were determined: mean pulmonary arterial (PA) pressure, pulmo-capillary resistance, teledyastolic right ventricular volume. The echocardiography showed significantly higher mean PA pressure values, an initial right ventricular dilatation and right ventricular wall hypertrophy. The indices of right heart catheterization were changed, too. The pulmo-capillary resistance was significantly increased. It was the most sensitive early parameter. The mean PA pressure was increased. There was no significant difference between the mean PA pressure determined by echocardiography and by right ventricular catheterization. A relationship between the degree of hypoxemia and the hemodynamic changes was found. The long development of COPD with mild respiratory failure leads to disorders in lung microcirculation and early changes in pulmonary hemodynamics. This process worsens the prognosis of COPD and the early prevention in the complex therapy of the disease is necessary.