Background: The obstructive sleep apnea-hypopnea syndrome (OSAHS) is characterized by repetitive upper airway obstructions during sleep, and it might cause cardiovascular complications such as myocardial infarction, arrhythmias, and systemic and pulmonary hypertension. Objectives: We investigated the acute effects of automatic continuous positive airway pressure ( automated CPAP) on blood pressure in patients with OSAHS and hypertension. Methods: Polysomnography was used and ambulatory blood pressure measurements were done in 12 patients with OSAHS. Blood pressure and heart rate were measured at night ( 10 p. m. to 6 a. m.) and during the day ( 6 a. m. to 10 p. m.). During these periods systolic, diastolic and mean blood pressure and heart rate of the patients on the diagnostic day were compared with those on the treatment day. Results: Patients had moderate or severe OSAHS; their mean age was 52.8 +/- 4.2 years. Systolic, diastolic and mean blood pressure and heart rate between the diagnostic and treatment day were not significantly different. Standard deviations of all these parameters during the night of the treatment day (9.1 +/- 4.5, 7.5 +/- 3.3, 8.0 +/- 3.0 mm Hg, and 4.8 +/- 1.5 beats/min, respectively) were significantly lower than during the night of the diagnostic day (12.6 +/- 4.9 mm Hg, p = 0.023, 10.8 +/- 3.5 mm Hg, p = 0.004, 11.6 +/- 4.4 mm Hg, p = 0.006 and 6.9 +/- 1.6 beats/min, p = 0.003, respectively). We did not find similar results during daytime periods. Conclusions: Automated CPAP therapy in patients with sleep apnea and hypertension did not decrease systolic and diastolic blood pressures and heart rates acutely. However, it might reduce the variability of these parameters during sleep in patients, but not during the day. It might be suggested that automated CPAP reduces cardiovascular morbidity of OSAHS via stabilizing heart rate and blood pressure during sleep. Copyright (C) 2005 S. Karger AG, Basel.