OSAHS raises the mean 24-hour blood pressure (BP) by 5mmHg to 10mmHg and the blood pressure increase is more in those patients with >20% arterial oxygen desaturation per hour of sleep. This rise in mean BP can lead to 20% increased risk of myocardial infarctions and 40% increased risk of stroke. The rise in BP is a result of surges of BP accompanying each arousal when the apnoea/hypoponea episode is terminated and as a result of an increased 24-hour sympathetic tone due to the repetitive episodes of hypoxaemia. These changes are associated with increased urinary levels of catecholamines, especially in those with more severe OSAHS, which reduces with continuous positive airway pressure (CPAP) treatment. OSAHS and hypertension frequently co- exist. It is estimated that about 20% of patients with hypertension may have OHSA and in people with OSAHS the prevalence of hypertension could be 50%.