Seventeen patients who were suspected of having hydrocephalus, because of ventricular dilation from various causes, were included in this study of the pathophysiologic basis of the appearance of pressure waves (PWs). Pressure waves accompanied by apnoeas originated in arousal responses in the resting state of these patients. Frequent fits of apnoea were included in the entire sleep apnoea syndrome. Most pressure waves characteristically appeared in the state of non-REM sleep. During the appearance of such pressure oscillations, intracranial pressure was elevated transiently. This coincidence in the appearance of pressure oscillations with sleep apnoea was the most characteristic pathophysiologic result from this polygraphic study of the patients.