Two groups of patients with nocturnal arterial oxygen desaturation were compared. The degree of nocturnal oxygen desaturation, as reflected by the percentage of total sleep time spent with an oxygen saturation less than 90 and 80%, was similar in patients with the obstructive sleep apnoea syndrome (OSAS) and in those with nocturnal hypoventilation (NH) secondary to restrictive chest wall disease. Systemic hypertension was present in 16 of the 24 OSAS patients but in only 6 of the 24 with NH (p less than 0.005). Multiple regression analysis demonstrated that this difference remained significant even after adjustment for age, sex, weight and history of smoking. It is likely that factors other than nocturnal hypoxaemia are important in the aetiology of systemic hypertension in patients with sleep-related breathing disorders.