"Cor pulmonale" is a classic feature of the "Pickwickian syndrome". Earlier studies have reported a high prevalence of pulmonary hypertension (PH) in obstructive sleep apnoea (OSA) patients, but this has not been confirmed by recent studies with a more adequate methodology, including larger groups of patients. The first part of this review is devoted to the prevalence of PH in OSA; most recent studies agree on prevalence of 15-20%. The second (and major) part of the study deals with the causes and mechanisms of PH in OSA. Pulmonary hypertension is rarely observed in the absence of day-time hypoxaemia, and the severity of nocturnal events (apnoea index (AI), apnoea+ hypopnoea index (AHI) does not appear to be the determining factor of PH. Diurnal arterial blood gas disturbances and PH are most often explained by the presence of severe obesity (obesity-hypoventilation syndrome) and, principally, by association of OSA with chronic obstructive pulmonary disease (the so called "overlap syndrome"). Bronchial obstruction is generally of mild-to-moderate degree and may be asymptomatic. The final part of the review analyses the therapeutic consequences of the presence of PH in OSA patients. Pulmonary hypertension, which is generally mild-to-moderate, does not need a specific treatment. When nasal continuous positive airway pressure (CPAP) fails to correct sleep-related hypoxaemia, supplementary oxygen must be administered. In patients with marked daytime hypoxaemia (arterial oxygen tension (Pa,O2), < or = 7.3 kPa (55 mmHg) conventional O2 therapy (nocturnal + diurnal) is required.