In order to describe variation in AP and ICP during OSA, six patients with severe OSA were examined, with determination of ICP, AP, CVP, respiration, tcPO2, tcPCO2, and nocturnal sleep polygraphy. During apnea, elevations of AP and ICP were observed, related to the apneic episodes. The elevations in pressure were only observed in relation to apneic episodes. While awake, none of the patients showed pressure elevations. There were highly significant correlations between duration of apnea and variation in AP and ICP and between variations in AP and ICP. Values for ICP while awake were above normal (greater than 15 mm Hg; intracranial hypertension) in four of six patients. Morning ICP was higher than evening ICP. Systolic, mean, and diastolic ICP and AP increased during sleep above awake values. The ICP increased during NREM stages 1 to 4, and the highest values were observed during REM sleep. Vascular response was not changed during REM sleep, and the higher ICP during REM could solely be explained by the longer apneas during REM sleep. The CPP decreased during apnea.