In 10 patients with sleep apnoea syndrome with and without systemic hypertension, non-invasive ambulatory 24-hour blood pressure monitoring was performed (8 patients with obstructive sleep apnoea, 2 patients with central sleep apnoea). Aim of the study was to investigate disturbances of circadian blood pressure rhythm and to correlate nightly decline in blood pressure with the respiratory events. Six patients have a history of systemic hypertension. In 6 patients (4 patients with and 2 patients without hypertension), the circadian rhythm was attenuated with a reduced nightly decrease or an increase in blood pressure. No correlations were found between nightly blood pressure changes and polysomnographic parameters.