A marked transient increase in blood pressure can occur at the end of each apneic period in patients with sleep apnea syndrome (SAS) and SAS may be a risk factor for cerebrovascular disease. To estimate blood pressure variability during apnea arterial blood pressure was directly and continuously recorded, and the transient increase in blood pressure in each sleep stage was assessed. Polysomnography was done in 5 men with SAS: arterial blood pressure, oxygen saturation, and respiratory curves were recorded. The maximum arterial blood pressure during the apneic period was compared with that at the end of apnea. The transient increase in blood pressure was 32.2 +/- 5.8 mmHg (mean +/- S.E.) for systolic pressure and 18.2 +/- 2.1 mmHg for diastolic pressure. During REM sleep, the values were 38.8 +/- 6.6 mmHg for systolic pressure and 23.4 +/- 2.2 mmHg for diastolic pressure. The increase in arterial blood pressure was significantly higher during REM sleep than during N-REM sleep (p < 0.05). This wide variation in blood pressure suggests that SAS is a risk factor for cerebrovascular disease, and that SAS may promote essential hypertension.