Apnoea with associated fall in arterial oxygen tension results in increased blood pressure and a striking surge in sympathetic activity, which can be measured as high catecholamine levels or increase in muscle sympathetic nerve activity. Following the termination of apnoea with resumption of breathing, sympathetic nerve activity decreases and blood pressure returns to lower values. Sympathetic mediated alternations in peripheral vascular resistance best explain these findings. Hypertension during wakefulness in untreated patients with apnoea is also associated with high sympathetic nervous system activity. Nasal continuous positive airway pressure (CPAP) has been shown to lower blood pressure in some hypertensive obstructive sleep apnoea (OSA) patients. Recently, previously untreated OSA patients exhibiting awake sympathetic hyperexcitation demonstrated striking attentuation of the response following initiation of effective CPAP therapy. Accordingly, the common problem of systemic hypertension found in untreated OSA appears to be mediated by sympathetic excitation and responds to effective CPAP therapy.