Obstructive sleep apnoea syndrome (OSAS) is a clinical disorder associated with hypertension; recent guidelines suggest that it should be considered as one of the causes of refractory hypertension. Good clinical practice indicates that a patient with suspected sleep apnoea should undergo 24 h blood pressure (BP) monitoring as well as polysomnography. Several studies have investigated a causal role of OSAS in hypertension and if these two conditions were linked by some possible confounding factor, such as obesity; recently, it has been observed that OSAS is an independent risk factor for hypertension and predisposes to a markedly increased incidence of cardiovascular disease. The physiopathological link between OSAS and high BP is not clear yet. Nevertheless it is possible to propose the hypothesis of a causal role of hypoxia, which characterizes sleep apnoea. In turn, hypoxia causes sympathetic activation, systemic vasoconstriction, endothelial dysfunction and increased serum leptin levels. All these factors may exacerbate the hypertensive state. Therapeutic strategies for hypertensive patients with OSAS include: continuous positive airway pressure (CPAP) treatment, which can reduce BP values and improve the autonomic control of BP and heart rate, and weight loss in obese patients, who represent about 70% of all patients with OSAS. In contrast, a pharmacological approach to treat sleep apnoea in hypertensives is not worthwhile at present. In conclusion, identification and treatment of OSAS in hypertensive patients is important to reduce the cardiovascular risk of these subjects.