We examined circadian blood pressure rhythm before and two days after onset of n-CPAP therapy in 50 patients, suffering from polysomnograhpically ensured severe obstructive sleep-apnea-syndrome. Measurements were performed using a non-invasive blood pressure registration. Patients with normal circadian blood pressure conditions and those with disturbed circadian rhythm could be differentiated. N-CPAP therapy may normalize circadian blood pressure conditions in some patients.