Majority of subjects with severe COPD develop mild pulmonary hypertension. Those patients usually present with diurnal, resting hypoxaemia. Further hypoxaemic dips during sleep probably contribute to development permanent pulmonary hypertension. However, separation of effects of diurnal from nocturnal hypoxaemia is difficult. Correlations between severity of nocturnal hypoxaemia and pulmonary arterial pressure and pulmonary vascular resistance were week. There was no correlation between time spent in desaturation and pulmonary arterial pressure and pulmonary resistance.