To determine the frequency and correlates of pulmonary hypertension in sleep-disordered breathing, pulmonary artery pressure, lung function and arterial blood gases were measured in 100 consecutive patients with obstructive sleep apnoea (OSA) (respiratory disturbance index (RDI) of > 20 episodes.h-1). Twenty six of the patients had significant chronic airflow limitation (CAL). Overall, 42% of patients had awake pulmonary artery pressure > 20 mmHg. Patients with pulmonary hypertension were older, had higher arterial carbon dioxide tension (PaCO2), lower arterial oxygen tension (PaO2) and lower forced expiratory volume in one second (FEV1) values compared with normotensive patients. Pao2, PaCO2 and FEV1 were correlated with the levels of pulmonary artery pressure (correlation coefficient (r2) 0.50, 0.46 and 0.49, respectively). These three factors combined could explain 33% of the variability in pulmonary artery pressure. Six patients had pulmonary hypertension despite a PaO2 in excess of 10.7 kPa (80 mmHg). We conclude that pulmonary hypertension is common in patients with moderate and severe sleep apnoea, especially those with coexisting chronic airflow limitation. The presence of daytime hypoxaemia is not a prerequisite in the development of pulmonary hypertension in these patients.