Pulmonary hypertension during exercise is common in severe chronic obstructive pulmonary disease (COPD). It was hypothesised that the use of the endothelin-receptor antagonist bosentan can improve cardiopulmonary haemodynamics during exercise, thus increasing exercise tolerance in patients with severe COPD. In the present double-blind, placebo-controlled study, 30 patients with severe or very severe COPD were randomly assigned in a 2:1 ratio to receive either bosentan or placebo for 12 weeks. The primary end-point was change in the 6-min walking distance. Secondary end-points included changes in health-related quality of life, lung function, cardiac haemodynamics, maximal oxygen uptake and pulmonary perfusion patterns. Compared with placebo, patients treated with bosentan during 12 weeks showed no significant improvement in exercise capacity as measured by the 6-min walking distance (mean+/-SD 331+/-123 versus 329+/-94 m). There was no change in lung function, pulmonary arterial pressure, maximal oxygen uptake or regional pulmonary perfusion pattern. In contrast, arterial oxygen pressure dropped, the alveolar-arterial gradient increased and quality of life deteriorated significantly in patients assigned bosentan. The oral administration of the endothelin receptor antagonist bosentan not only failed to improve exercise capacity but also deteriorated hypoxaemia and functional status in severe chronic obstructive pulmonary disease patients without severe pulmonary hypertension at rest.